Rollup merge of #122247 - notriddle:notriddle/search-unbox-limit, r=GuillaumeGomez

rustdoc-search: depth limit `T<U>` -> `U` unboxing

Profiler output:
https://notriddle.com/rustdoc-html-demo-9/search-unbox-limit/ (the only significant change is that one of the `rust` tests went from 378416ms to 16ms).

This is a performance enhancement aimed at a problem I found while using type-driven search on the Rust compiler. It is caused by [`Interner`], a trait with 41 associated types, many of which recurse back to `Self` again.

This caused search.js to struggle. It eventually terminates, after about 10 minutes of turning my PC into a space header, but it's doing `41!` unifications and that's too slow.

[`Interner`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.Interner.html

Author: matthiaskrgr