MS ABI: Update the thunk linkage computation

As suggested by Reid:

 - class has GVA_Internal linkage -> internal
 - thunk has return adjustment -> weak_odr, to handle evil corner case [1]
 - all other normal methods -> linkonce_odr

 1. Evil corner case:

  struct Incomplete;
  struct A { int a; virtual A *bar(); };
  struct B { int b; virtual B *foo(Incomplete); };
  struct C : A, B { int c; virtual C *foo(Incomplete); };
  C c;

Here, the thunk for C::foo() will be emitted when C::foo() is defined, which
might be in a different translation unit, so it needs to be weak_odr.

Differential Revision: http://reviews.llvm.org/D3992

llvm-svn: 210368

clang/lib/CodeGen/CGVTables.cpp

@@ -382,12 +382,14 @@ void CodeGenVTables::emitThunk(GlobalDecl GD, const ThunkInfo &Thunk,
     // FIXME: Do something better here; GenerateVarArgsThunk is extremely ugly.
     if (!UseAvailableExternallyLinkage) {
       CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, Thunk);
-      CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable);
+      CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
+                                      !Thunk.Return.isEmpty());
     }
   } else {
     // Normal thunk body generation.
     CodeGenFunction(CGM).GenerateThunk(ThunkFn, FnInfo, GD, Thunk);
-    CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable);
+    CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
+                                    !Thunk.Return.isEmpty());
   }
 }