On 32 bit windows, mangle stdcall and fastcall decls in clang.

This removes the dependency on the llvm mangler doing it for us. In isolation,
the benefit is that the testing of what mangling is applied is all in one place:
(C, C++) X (Itanium, Microsoft) are all handled by clang.

This also gives me hope that in the future the llvm mangler (and llvm-ar) will
not depend on TargetMachine.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192762 91177308-0d34-0410-b5e6-96231b3b80d8
Remove a workaround for PR16406 from the vtables-virtual-inheritance test

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192786 91177308-0d34-0410-b5e6-96231b3b80d8