Use castAs instead of cast in thunk generation

Calling convention attributes can add sugar to methods that we have to
look through.  This fixes an assertion failure in the provided test
case.

llvm-svn: 192496

clang/lib/CodeGen/CGVTables.cpp

@@ -481,7 +481,7 @@ void CodeGenVTables::maybeEmitThunkForVTable(GlobalDecl GD,
   // We can't emit thunks for member functions with incomplete types.
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
   if (!CGM.getTypes().isFuncTypeConvertible(
-                                cast<FunctionType>(MD->getType().getTypePtr())))
+           MD->getType()->castAs<FunctionType>()))
     return;
 
   emitThunk(GD, Thunk, /*ForVTable=*/true);

clang/test/CodeGenCXX/microsoft-abi-thunks.cpp

@@ -116,3 +116,18 @@ struct H : E {
 H h;
 
 // FIXME: Write vtordisp adjusting thunk tests
+
+namespace CrashOnThunksForAttributedType {
+// We used to crash on this because the type of foo is an AttributedType, not
+// FunctionType, and we had to look through the sugar.
+struct A {
+  virtual void __stdcall foo();
+};
+struct B {
+  virtual void __stdcall foo();
+};
+struct C : A, B {
+  virtual void __stdcall foo();
+};
+C c;
+}