Fix crash with x86_64-pc-win32-macho target. <rdar://problem/17235840>

The changes in r204978 broke win32-macho targets. There were checks added for
MSVC and Itanium environments as special cases, and win32-macho needs to be
treated the same way.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210584 91177308-0d34-0410-b5e6-96231b3b80d8