Emit thunk function for specific ABI on WebAssembly. (#6)

Changed to make thunk to convert thin-to-thick and non-throws-to-throws. 
We needs it on WebAssembly host because WASM checks number of arguments strictly for indirect function call.

This patch allows you to run the Swift code below.

```swift
func f(_ a: (Int) -> Void) {
    g(a)
}

func g(_ b: (Int) throws -> Void) {
    try! b(1)
}

f { _ in }
```

lib/SIL/TypeLowering.cpp

@@ -2609,6 +2609,14 @@ TypeConverter::checkFunctionForABIDifferences(SILModule &M,
       return ABIDifference::NeedsThunk;
   }
 
+  // There is no ABI compatibility between non-throws and throws on WebAssembly,
+  // so need thunk.
+  if (M.getASTContext().LangOpts.Target.isOSBinFormatWasm()) {
+    if (!fnTy1->hasErrorResult() && fnTy2->hasErrorResult()) {
+      return ABIDifference::NeedsThunk;
+    }
+  }
+
   auto rep1 = fnTy1->getRepresentation(), rep2 = fnTy2->getRepresentation();
   if (rep1 != rep2) {
     if (rep1 == SILFunctionTypeRepresentation::Thin &&
@@ -2620,8 +2628,14 @@ TypeConverter::checkFunctionForABIDifferences(SILModule &M,
       } else {
         return ABIDifference::CompatibleRepresentation_ThinToThick;
       }
-    }
 
+      // There is no ABI compatibility between thin and thick on WebAssembly,
+      // so need thunk.
+      if (M.getASTContext().LangOpts.Target.isOSBinFormatWasm()) {
+        return ABIDifference::NeedsThunk;
+      }
+      return ABIDifference::ThinToThick;
+    }
     return ABIDifference::NeedsThunk;
   }