The test is currently marked XFAIL for mingw environments, but in
the future, it will start passing. (I'll update this with a reference
to the mingw-w64 commit once I've pushed it.)
The CI environment will probably be upgraded to a state where it is
passing only after 14.x is branched in the llvm-project monorepo.
If we'd just go from having an XFAIL to no marking at all (when CI is
passing), we'd have to update both main and 14.x branches in sync
exactly when the CI runners are updated to a newer version.
Instead, mark the test as temporarily unsupported (so it doesn't
cause failed builds when the CI environment is updated); after the
CI environments are upgraded to such a state, we can remove the
UNSUPPORTED marking to start requiring it to pass on the main branch,
without needing to synchronize that change to anything else.