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.