[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
5 files changed