Auto merge of #75008 - eddyb:rmeta-indexed-trait-impls, r=nikomatsakis

rustc_metadata: track the simplified Self type for every trait impl.

For the `traits_impls_of` query, we index the impls by `fast_reject::SimplifiedType` (a "shallow type"), which allows some simple cases like `impl Trait<..> for Foo<..>` to be efficiently iterated over, by e.g. `for_each_relevant_impl`.

This PR encodes the `fast_reject::SimplifiedType` cross-crate to avoid needing to deserialize the `Self` type of every `impl` in order to simplify it - the simplification itself should be cheap, but the deserialization is less so.

We could go further from here and make loading the list of impls lazy, for a given simplified `Self` type, but that would have more complicated implications for performance, and this PR doesn't do anything in that regard.

r? @nikomatsakis cc @Mark-Simulacrum

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