Auto merge of #53162 - QuietMisdreavus:crouching-impl-hidden-trait, r=GuillaumeGomez

rustdoc: collect trait impls as an early pass

Fixes https://github.com/rust-lang/rust/issues/52545, fixes https://github.com/rust-lang/rust/issues/41480, fixes https://github.com/rust-lang/rust/issues/36922

Right now, rustdoc pulls all its impl information by scanning a crate's HIR for any items it finds. However, it doesn't recurse into anything other than modules, preventing it from seeing trait impls that may be inside things like functions or consts. Thanks to https://github.com/rust-lang/rust/pull/53002, now these items actually *exist* for rustdoc to see, but they still weren't getting collected for display.

But there was a secret. Whenever we pull in an item from another crate, we don't have any of its impls in the local HIR, so instead we ask the compiler for *everything* and filter out after the fact. This process is only triggered if there's a cross-crate re-export in the crate being documented, which can sometimes leave this info out of the docs. This PR instead moves this collection into an early pass, which occurs immediately after crate cleaning, so that that collection occurs regardless. In addition, by including the HIR's own `trait_impls` in addition to the existing `all_trait_implementations` calls, we can collect all these tricky trait impls without having to scan for them!