Some targets (e.g. Darwin) might have the Win32 API available, but they
do not use MSVC CRT. Assume _LIBCPP_MSVCRT only when _MSC_VER is available
and __MINGW32__ isn't defined.
Differential Revision: https://reviews.llvm.org/D34588
rdar://problem/32628786
llvm-svn: 308225