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.