This change adds support for __builtin_ms_va_list, a GCC extension for
variadic ms_abi functions. The existing __builtin_va_list support is
inadequate for this because va_list is defined differently in the Win64
ABI vs. the System V/AMD64 ABI.
Depends on D1622.
Reviewers: rsmith, rnk, rjmccall
Differential Revision: http://reviews.llvm.org/D1623