MS Compat: interpose vadefs.h to fix definitions of _crt_va_{start,end,arg} (PR21247)

Differential revision: http://reviews.llvm.org/D5784

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219740 91177308-0d34-0410-b5e6-96231b3b80d8