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.

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[lcnr]

r? lcnr

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[cordx56]

@rustbot review