Rollup merge of #119997 - GuillaumeGomez:fix-stripped-impl-on-ty-alias, r=notriddle

Fix impl stripped in rustdoc HTML whereas it should not be in case the impl is implemented on a type alias

Fixes #119015.

I talked about it a bit with ```@petrochenkov.``` They might change what `EffectiveVisibilities` return for impl items like this one and make them not only reachable but also re-exported, which would fix this case. It could also potentially break other things, so it'll be done whenever they can and then we can check together.

Surprisingly, this fix is making rustdoc even closer to rustc in term of errors (the CI currently fails because currently accepted broken codes aren't working anymore with this change). Not sure exactly why though. This is linked to https://github.com/rust-lang/rust/pull/110631 from what I could find.

So either I'm missing something here, or we consider it's ok and we consider the failing tests as "should fail" and I'll update `rustdoc-ui` ones.

r? ```@notriddle```

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