Allow EmitVAArg() to promote types and use this to fix some N32/N64 vararg issues for Mips.

Summary:
With this patch, passing a va_list to another function and reading 10 int's from
it works correctly on a big-endian target.

Based on a pair of patches by David Chisnall, one of which I've reworked
for the current trunk.

Reviewers: theraven, atanasyan

Reviewed By: theraven, atanasyan

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6248

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222339 91177308-0d34-0410-b5e6-96231b3b80d8