Fix supertrait associated type unsoundness
### What?
Object safety allows us to name `Self::Assoc` associated types in certain positions if they come from our trait or one of our supertraits. When this check was implemented, I think it failed to consider that supertraits can have different args, and it was only checking def-id equality.
This is problematic, since we can sneak different implementations in by implementing `Supertrait<NotActuallyTheSupertraitSubsts>` for a `dyn` type. This can be used to implement an unsound transmute function. See the committed test.
### How do we fix it?
We consider the whole trait ref when checking for supertraits. Right now, this is implemented using equality *without* normalization. We erase regions since those don't affect trait selection.
This is a limitation that could theoretically affect code that should be accepted, but doesn't matter in practice -- there are 0 crater regression. We could make this check stronger, but I would be worried about cycle issues. I assume that most people are writing `Self::Assoc` so they don't really care about the trait ref being normalized.
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### What is up w the stacked commit
This is built on top of https://github.com/rust-lang/rust/pull/122804 though that's really not related, it's just easier to make this modification with the changes to the object safety code that I did in that PR. The only thing is that PR may make this unsoundness slightly easier to abuse, since there are more positions that allow self-associated-types -- I am happy to stall that change until this PR merges.
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Fixes#126079
r? lcnr
