Switch from XFAIL to UNSUPPORTED; NFC

This test is currently marked as XFAIL for Windows, but running the
test with a debug build of clang-repl.exe crashes with a modal system
dialog. This switches the test to UNSUPPORTED instead. This makes the
test behavior less onerous for those of us doing Debug builds, at the
expense of a minor bit of coverage if the test were ever to start
passing unexpectedly on Windows (which seems like an unlikely event).