[libc++] Use _LIBCPP_ABI_MICROSOFT instead of _MSC_VER

_LIBCPP_ABI_MICROSOFT is more appropriate to use here, since the
conditionals are controlling Microsoft mangling. It wasn't used
originally since it didn't exist at the time.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300743 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed