Add RTN support to rustdoc

[compiler-errors]

This adds support to rustdoc and rustdoc-json for rendering (..) RTN (return type notation) style generics.

---

Cleaning rustc_middle::ty::Ty is not correct still, though, and ends up rendering a function like:

pub fn foreign<T: Foreign<bar(..): Send>>()
where
    <T as Foreign>::bar(..): 'static,
    T::bar(..): Sync,

Into this:

pub fn foreign<T>()
where
    T: Foreign,
    impl Future<Output = ()>: Send + 'static + Sync,

This is because clean_middle_ty doesn't actually have sufficient context about whether the RPITIT is in its "defining scope" or not, so we don't know if the type was originally written like -> impl Trait or with RTN like T::method(..).

Closes #123996

[rustbot]

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[rustbot]

rustdoc-json-types is a public (although nightly-only) API. If possible, consider changing src/librustdoc/json/conversions.rs; otherwise, make sure you bump the FORMAT_VERSION constant.

cc @CraftSpider, @aDotInTheVoid, @Enselic, @obi1kenobi

Some changes occurred in tests/rustdoc-json

cc @aDotInTheVoid

[bors]

☔ The latest upstream changes (presumably #138155) made this pull request unmergeable. Please resolve the merge conflicts.

[aDotInTheVoid]

This is taking place at the same time as #138109, which also (currently) makes the 40->41 change. Once one of these merges, the other should rebase then bump again. See #94591 for details.