[swiftc (121 vs. 5184)] Add crasher in swift::ASTVisitor

Add test case for crash triggered in `swift::ASTVisitor`.

Current number of unresolved compiler crashers: 121 (5184 resolved)

Assertion failure in [`lib/Sema/CSApply.cpp (line 2599)`](https://github.com/apple/swift/blob/master/lib/Sema/CSApply.cpp#L2599):

```
Assertion `conforms && "Type does not conform to protocol?"' failed.

When executing: swift::Expr *<anonymous namespace>::ExprRewriter::visitArrayExpr(swift::ArrayExpr *)
```

Assertion context:

```
      bool conforms = tc.conformsToProtocol(arrayTy, arrayProto,
                                            cs.DC,
                                            ConformanceCheckFlags::InExpression,
                                            &conformance);
      (void)conforms;
      assert(conforms && "Type does not conform to protocol?");

      // Call the witness that builds the array literal.
      // FIXME: callWitness() may end up re-doing some work we already did
      // to convert the array literal elements to the element type. It would
      // be nicer to re-use them.
```
Stack trace:

```
#0 0x00000000031d10e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x31d10e8)
#1 0x00000000031d1936 SignalHandler(int) (/path/to/swift/bin/swift+0x31d1936)
#2 0x00007f9d8070c330 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x10330)
#3 0x00007f9d7eecac37 gsignal /build/eglibc-oGUzwX/eglibc-2.19/signal/../nptl/sysdeps/unix/sysv/linux/raise.c:56:0
#4 0x00007f9d7eece028 abort /build/eglibc-oGUzwX/eglibc-2.19/stdlib/abort.c:91:0
#5 0x00007f9d7eec3bf6 __assert_fail_base /build/eglibc-oGUzwX/eglibc-2.19/assert/assert.c:92:0
#6 0x00007f9d7eec3ca2 (/lib/x86_64-linux-gnu/libc.so.6+0x2fca2)
#7 0x0000000000c3028a swift::ASTVisitor<(anonymous namespace)::ExprRewriter, swift::Expr*, void, void, void, void, void>::visit(swift::Expr*) (/path/to/swift/bin/swift+0xc3028a)
#8 0x0000000000c230c0 (anonymous namespace)::ExprRewriter::walkToExprPost(swift::Expr*) (/path/to/swift/bin/swift+0xc230c0)
#9 0x0000000000c34cf0 (anonymous namespace)::ExprWalker::walkToExprPost(swift::Expr*) (/path/to/swift/bin/swift+0xc34cf0)
#10 0x0000000000d659d5 swift::Expr::walk(swift::ASTWalker&) (/path/to/swift/bin/swift+0xd659d5)
#11 0x0000000000c1fb22 swift::constraints::ConstraintSystem::applySolution(swift::constraints::Solution&, swift::Expr*, swift::Type, bool, bool, bool) (/path/to/swift/bin/swift+0xc1fb22)
#12 0x0000000000b8da04 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) (/path/to/swift/bin/swift+0xb8da04)
#13 0x0000000000c44105 (anonymous namespace)::FailureDiagnosis::typeCheckChildIndependently(swift::Expr*, swift::Type, swift::ContextualTypePurpose, swift::OptionSet<TCCFlags, unsigned int>, swift::ExprTypeCheckListener*) (/path/to/swift/bin/swift+0xc44105)
#14 0x0000000000c55393 (anonymous namespace)::FailureDiagnosis::typeCheckArgumentChildIndependently(swift::Expr*, swift::Type, (anonymous namespace)::CalleeCandidateInfo const&, swift::OptionSet<TCCFlags, unsigned int>) (/path/to/swift/bin/swift+0xc55393)
#15 0x0000000000c5126b (anonymous namespace)::FailureDiagnosis::visitApplyExpr(swift::ApplyExpr*) (/path/to/swift/bin/swift+0xc5126b)
#16 0x0000000000c4ce0f swift::ASTVisitor<(anonymous namespace)::FailureDiagnosis, bool, void, void, void, void, void>::visit(swift::Expr*) (/path/to/swift/bin/swift+0xc4ce0f)
#17 0x0000000000c3cab6 swift::constraints::ConstraintSystem::diagnoseFailureForExpr(swift::Expr*) (/path/to/swift/bin/swift+0xc3cab6)
#18 0x0000000000c43210 swift::constraints::ConstraintSystem::salvage(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::Expr*) (/path/to/swift/bin/swift+0xc43210)
#19 0x0000000000b8aecf swift::TypeChecker::solveForExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem&, llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>) (/path/to/swift/bin/swift+0xb8aecf)
#20 0x0000000000b8d96e swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) (/path/to/swift/bin/swift+0xb8d96e)
#21 0x0000000000b913a7 swift::TypeChecker::typeCheckCondition(swift::Expr*&, swift::DeclContext*) (/path/to/swift/bin/swift+0xb913a7)
#22 0x0000000000b91bba swift::TypeChecker::typeCheckExprPattern(swift::ExprPattern*, swift::DeclContext*, swift::Type) (/path/to/swift/bin/swift+0xb91bba)
#23 0x0000000000bd4031 swift::TypeChecker::coercePatternToType(swift::Pattern*&, swift::DeclContext*, swift::Type, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, swift::TypeLoc) (/path/to/swift/bin/swift+0xbd4031)
#24 0x0000000000bd38f4 swift::TypeChecker::coercePatternToType(swift::Pattern*&, swift::DeclContext*, swift::Type, swift::OptionSet<swift::TypeResolutionFlags, unsigned int>, swift::GenericTypeResolver*, swift::TypeLoc) (/path/to/swift/bin/swift+0xbd38f4)
#25 0x0000000000b950bb swift::TypeChecker::typeCheckBinding(swift::Pattern*&, swift::Expr*&, swift::DeclContext*)::BindingListener::appliedSolution(swift::constraints::Solution&, swift::Expr*) (/path/to/swift/bin/swift+0xb950bb)
#26 0x0000000000b8da9c swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::TypeLoc, swift::ContextualTypePurpose, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>, swift::ExprTypeCheckListener*, swift::constraints::ConstraintSystem*) (/path/to/swift/bin/swift+0xb8da9c)
#27 0x0000000000b90b35 swift::TypeChecker::typeCheckBinding(swift::Pattern*&, swift::Expr*&, swift::DeclContext*) (/path/to/swift/bin/swift+0xb90b35)
#28 0x0000000000b91535 swift::TypeChecker::typeCheckStmtCondition(llvm::MutableArrayRef<swift::StmtConditionElement>&, swift::DeclContext*, swift::Diag<>) (/path/to/swift/bin/swift+0xb91535)
#29 0x0000000000c02771 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0xc02771)
#30 0x0000000000c024cc swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*) (/path/to/swift/bin/swift+0xc024cc)
#31 0x0000000000c01db6 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*) (/path/to/swift/bin/swift+0xc01db6)
#32 0x0000000000c15d0a swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) (/path/to/swift/bin/swift+0xc15d0a)
#33 0x0000000000938c66 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0x938c66)
#34 0x000000000047ece5 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47ece5)
#35 0x000000000047db7f swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47db7f)
#36 0x000000000044509a main (/path/to/swift/bin/swift+0x44509a)
#37 0x00007f9d7eeb5f45 __libc_start_main /build/eglibc-oGUzwX/eglibc-2.19/csu/libc-start.c:321:0
#38 0x0000000000442816 _start (/path/to/swift/bin/swift+0x442816)
```

validation-test/compiler_crashers/28494-conforms-type-does-not-conform-to-protocol.swift

@@ -0,0 +1,10 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+// REQUIRES: asserts
+if let[e]=5