declare_interior_mutable_const, borrow_interior_mutable_const: resolve <T as Trait>::AssocT projections

[GrigorenkoPV]

changelog: [declare_interior_mutable_const, borrow_interior_mutable_const]: resolve <T as Trait>::AssocT projections

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This came up during rust-lang/rust#130543 where we have <T as AtomicPrimitive>::Assoc = AtomicT instead of just AtomicT and clippy failed to resolve that properly.

This really needs a review, because

• I don't know if try_normalize_erasing_regions is the right thing to call here.
• I'm not sure if I peel off the correct amount of ValTree::Branch layers (I think I do).

Also, shouldn't this lint's infrastructure rely on Freeze trait (rust-lang/rust#121675) instead of hardcoding a list of known-to-be-interior-mutable types?

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Previously filed this in the main rust repo (rust-lang/rust#136369), was asked to do it here instead (rust-lang/rust#136369 (comment)).

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