Rollup merge of #137269 - dianne:fix-ref-pat-label-span, r=davidtwco

Pattern Migration 2024: properly label `&` patterns whose subpatterns are from macro expansions

See the failing test output in the first commit for an example of what this going wrong looks like. The error/lint diagnostic tries to point to just the `&` or `&mut` of reference patterns when labeling the causes, to make the output clearer (#134394). The trimming there wasn't quite right though: it used the interior of the reference pattern as a cutoff and extended backwards to find where to trim the pattern's span, but this breaks if the `&` and the interior are from different sources. This PR instead trims by starting at the start of the pattern and ending at the final character of the `&` (or `&mut`, `ref`, `ref mut`, or `mut`, depending on what the error/lint is labeling); that way, there's no opportunity for failure from mixing sources.

I'm not 100% happy with this approach, but I'm also not sure what the best practices are as far as hacky `SourceMap` munching goes, so please let me know if something else would be preferred.

Since `SourceMap::span_through_char` can't change the syntax context of the span, I've also removed a call to `Span::with_ctxt` (we care about the edition of the span in question since this is a hard error in Rust 2024). If we want to be extra safe in case that changes, I can re-add it or track error hardness separately in the `rust_2024_migration_desugared_pats` table.

Author: matthiaskrgr