Don't trap when passing non-POD arguments to variadic functions in MS-compatibility mode

Clang warns (treated as error by default, but still ignored in system headers)
when passing non-POD arguments to variadic functions, and generates a trap
instruction to crash the program if that code is ever run.

Unfortunately, MSVC happily generates code for such calls without a warning,
and there is code in system headers that use it.

This makes Clang not insert the trap instruction when in -fms-compatibility
mode, while still generating the warning/error message.

Differential Revision: http://reviews.llvm.org/D5492