[libcxx] [test] Change LIBCXX-WINDOWS-FIXME into XFAIL: msvc for cases that succeed in mingw configurations Add comments about the reasons for the XFAILs where there was none before. Differential Revision: https://reviews.llvm.org/D112211 GitOrigin-RevId: a21a6ed8c23ff16cd0971606f85a00f8262064fa