Rework how the disallowed qualifier in function type diagnostics are generated
This pull request fixes two independent issues:
1. When qualifiers of a function type ptr are in the wrong order and one of them is async/const (not permitted on function types), the diagnostic suggests removing the incorrect qualifier. Fixes https://github.com/rust-lang/rust/issues/142268, which is an issue created by https://github.com/rust-lang/rust/pull/133151. This is fixed by moving the check into `parse_fn_front_matter`, where better span information is available to generate the right suggestions.
2. When qualifiers of a function type ptr are in the wrong order and one of them is async/const (not permitted on function types), `cargo fix` crashes because "cannot replace slice of data that was already replaced". This is fixed by not generating a suggestion for the "wrong order" diagnostic if the "disallowed qualifier" diagnostic is triggered.
There is a commit with failing tests so the test diff is clearer
r? `@jdonszelmann`
4db9554358