[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

libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp

@@ -9,7 +9,11 @@
 // UNSUPPORTED: c++03
 // ADDITIONAL_COMPILE_FLAGS: -I %S/../../../../../../src/filesystem
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
+// This test relies on calling functions from the libcxx internal header
+// filesystem_common.h; the Windows implementation uses different
+// internals and doesn't provide the same set_file_times function as for
+// other platforms.
+// UNSUPPORTED: windows
 
 // <filesystem>