Make variable argument intrinsics behave correctly in a Win64 CC function.

Summary:
This change makes the variable argument intrinsics, `llvm.va_start` and
`llvm.va_copy`, and the `va_arg` instruction behave as they do on Windows
inside a `CallingConv::X86_64_Win64` function. It's needed for a Clang patch
I have to add support for GCC's `__builtin_ms_va_list` constructs.

Reviewers: nadav, asl, eugenis

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1622

llvm-svn: 245990