Win64: Silently ignore __stdcall, __fastcall, and __thiscall

MSVC doesn't warn on this. Users are expected to apply the WINAPI macro
to functions passed by pointer to the Win32 API, and this macro expands
to __stdcall. This means we end up with a lot of useless noisy warnings
about ignored calling conventions when compiling code with clang for
Win64.

llvm-svn: 230668