fix: defer const normalize in coherence mode

[cordx56]

I'm not confident that this fix is the best and desired in the long term point of view, but I'm confident that this does not do anything unsound.

Fixes #157937, by deferring normalization of associated const, except for anon, in coherence mode.
This fix prevents reporting impls, including associated const, are not overlapping.

[rustbot]

changes to the core type system

cc @lcnr

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @petrochenkov (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 18 candidates

[lcnr]

r? lcnr