[libcxx] [test] Convert an XFAIL: LIBCXX-WINDOWS-FIXME into XFAIL: msvc

This one already had a proper explanation why it fails, which is due to
differences by design in MSVC mode. This isn't a fixme, so degrade the
annotation to a more permanent "XFAIL: msvc" instead.

Differential Revision: https://reviews.llvm.org/D107758