[cling] Emit const variables only once

Otherwise they are emitted as interal and we get double-construction
and -destruction on the same memory address due to the way we promote
interal declarations in KeepLocalGVPass.

Fixes root-project#13429

interpreter/cling/test/CodeGeneration/const.C

@@ -0,0 +1,37 @@
+//------------------------------------------------------------------------------
+// CLING - the C++ LLVM-based InterpreterG :)
+//
+// This file is dual-licensed: you can choose to license it under the University
+// of Illinois Open Source License or the GNU Lesser General Public License. See
+// LICENSE.TXT for details.
+//------------------------------------------------------------------------------
+
+// RUN: cat %s |  %cling 2>&1 | FileCheck %s
+
+extern "C" int printf(const char*, ...);
+
+struct A { 
+  int val;
+  A(int v) : val(v) {
+    printf("A(%d), this = %p\n", val, this);
+  }
+  ~A() {
+    printf("~A(%d), this = %p\n", val, this);
+  }
+  int getVal() const { return val; }
+};
+
+const A a(1);
+// CHECK: A(1), this = [[PTR:0x[0-9a-f]+]]
+
+a.val
+// CHECK-NEXT: (const int) 1
+a.getVal()
+// CHECK-NEXT: (int) 1
+a.val
+// CHECK-NEXT: (const int) 1
+a.getVal()
+// CHECK-NEXT: (int) 1
+
+// CHECK-NEXT: ~A(1), this = [[PTR]]
+// CHECK-NOT: ~A

interpreter/llvm-project/clang/lib/AST/ASTContext.cpp

@@ -10831,6 +10831,17 @@ GVALinkage ASTContext::GetGVALinkageForFunction(const FunctionDecl *FD) const {
 
 static GVALinkage basicGVALinkageForVariable(const ASTContext &Context,
                                              const VarDecl *VD) {
+  // As an extension for interactive REPLs, make sure constant variables are
+  // only emitted once instead of LinkageComputer::getLVForNamespaceScopeDecl
+  // marking them as internal.
+  if (Context.getLangOpts().CPlusPlus &&
+      VD->getType().isConstQualified() &&
+      !VD->getType().isVolatileQualified() &&
+      !VD->isInline() &&
+      !isa<VarTemplateSpecializationDecl>(VD) &&
+      !VD->getDescribedVarTemplate())
+    return GVA_DiscardableODR;
+
   if (!VD->isExternallyVisible())
     return GVA_Internal;