Check for _MSC_VER before defining _LIBCPP_MSVCRT

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