Tighten diagnostics for calling conventions on variadic functions

Follow-up from r192240.

This makes it an error to use callee-cleanup conventions on variadic
functions, except for __fastcall and __stdcall, which we ignore with
a warning for GCC and MSVC compatibility.

Differential Revision: http://llvm-reviews.chandlerc.com/D1870

llvm-svn: 192308