[libcxx] [test] Explain an XFAIL LIBCXX-WINDOWS-FIXME and convert into UNSUPPORTED

This particular test relies on internal details from the libc++
filesystem implementation header, and those details are structured
differently in the implementation for Windows.

Differential Revision: https://reviews.llvm.org/D102357

GitOrigin-RevId: ec402f1d5da02dfbdc87d031ff99d7dbcada336f
1 file changed