Rollup merge of #104295 - compiler-errors:rpitit-generics-parity, r=eholk

Rollup merge of #104295 - compiler-errors:rpitit-generics-parity, r=eholk

Check generics parity before collecting return-position `impl Trait`s in trait

The only thing is that this duplicates the error message for number of generics mismatch, but we already deduplicate that error message in Cargo. I could add a flag to delay the error if the reviewer cares.

Fixes #104281

Also drive-by adds a few comments to the `collect_trait_impl_trait_tys` method, and removes an unused argument from `compare_number_of_generics`.