After this commit, `unsafe async fn ...` now suggests the `async unsafe` fix
instead of misunderstanding the issue.
This is not perfect for repeated keywords (`const async const`) and for
keywords that are misplaced after `extern "some abi"` because of the way
`check_fn_font_matter` works, but changing it breaks so many tests and
diagnostics it has been judged too high a cost for this PR.
690cbb79b3