Rollup merge of #118346 - compiler-errors:deeply-normalize-for-diagnostic, r=lcnr

Add `deeply_normalize_for_diagnostics`, use it in coherence

r? lcnr

Normalize trait refs used for coherence error reporting with `-Ztrait-solver=next-coherence`.

Two things:
1. I said before that we can't add this to `TyErrCtxt` because we compute `OverlapResult`s even if there are no diagnostics being emitted, e.g. for a reservation impl.
2. I didn't want to add this to an `InferCtxtExt` trait because I felt it was unnecessary. I don't particularly care about the API though.

Author: compiler-errors