Follow-up commit to r211657 which introduced these macros, but not for MSVC....

Follow-up commit to r211657 which introduced these macros, but not for MSVC. This turns out to break our freestanding tests on Windows when compiling in MSVC-compatible mode. It was decided (http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20141020/116942.html is the start of the thread) to support this as part of Clang's interface on all platforms.

llvm-svn: 220312