Turn error about fastcall variadic function into warning in MS mode (PR12535)

MSVC allows this and silently falls back to __cdecl for variadic functions.
This patch turns Clang's error into a warning in MS mode and adds a test
to make sure we generate correct code.

Differential Revision: http://llvm-reviews.chandlerc.com/D1861

llvm-svn: 192240