Rework how the disallowed qualifier in function type diagnostics are generated

[JonathanBrouwer]

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 invalid token removal suggested on unsafe const fn() type #142268, which is an issue created by Trim extra whitespace in fn ptr suggestion span #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