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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192308 91177308-0d34-0410-b5e6-96231b3b80d8