Rollup merge of #67059 - TommasoBianchi:dropck_fix_pr, r=pnkfelix

Fix too restrictive checks on Drop impls

Fixes #34426. Fixes #58311.

This PR completes and extends #59497 (which has been inactive for a while now).
The problem generating both issues was that when checking that the `Predicate`s of the `Drop` impl are exactly the same as the ones of the struct definition, the check was essentially performed by a simple `==` operator, which was not handling correctly HRTBs and involved `Fn` types.

The implemented solution relies on the `relate` machinery to more correctly equate `Predicate`s, and on `anonymize_late_bound_regions` to handle HRTB in a more general way. As the `Relate` trait currently is implemented only for `TraitPredicate` and `ProjectionPredicate` (and as they were the ones generating problems), `relate` is used only for them while for other `Predicate`s the equality check is kept. I'm currently considering whether it would make sense to implement the `Relate` trait also for all other `Predicate`s to render the proposed solution more general.

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