Rewrite on_unimplemented format string parser.

[mejrs]

This PR rewrites the format string parser for rustc_on_unimplemented and diagnostic::on_unimplemented. I plan on moving this code (and more) into the new attribute parsing system soon and wanted to PR it separately.

This PR introduces some minor differences though:

• rustc_on_unimplemented on trait implementations is no longer checked/used - this is actually never used (outside of some tests) so I plan on removing it in the future.
• for rustc_on_unimplemented, it introduces the {This} argument in favor of {ThisTraitname} (to be removed later). It'll be easier to parse.
• for rustc_on_unimplemented, Self can now consistently be used as a filter, rather than just _Self. It used to not match correctly on for example Self = "[{integer}]"
• Some error messages now have better spans.

Fixes #130627

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This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.