Auto merge of #109010 - compiler-errors:rtn, r=eholk

Initial support for return type notation (RTN)

See: https://smallcultfollowing.com/babysteps/blog/2023/02/13/return-type-notation-send-bounds-part-2/

1. Only supports `T: Trait<method(): Send>` style bounds, not `<T as Trait>::method(): Send`. Checking validity and injecting an implicit binder for all of the late-bound method generics is harder to do for the latter.
    * I'd add this in a follow-up.
3. ~Doesn't support RTN in general type position, i.e. no `let x: <T as Trait>::method() = ...`~
    * I don't think we actually want this.
5. Doesn't add syntax for "eliding" the function args -- i.e. for now, we write `method(): Send` instead of `method(..): Send`.
    * May be a hazard if we try to add it in the future. I'll probably add it in a follow-up later, with a structured suggestion to change `method()` to `method(..)` once we add it.
7. ~I'm not in love with the feature gate name 😺~
    * I renamed it to `return_type_notation` ✔️

Follow-up PRs will probably add support for `where T::method(): Send` bounds. I'm not sure if we ever want to support return-type-notation in arbitrary type positions. I may also make the bounds require `..` in the args list later.

r? `@ghost`

compiler/rustc_feature/src/active.rs

@@ -495,6 +495,8 @@ declare_features! (
     (active, repr_simd, "1.4.0", Some(27731), None),
     /// Allows return-position `impl Trait` in traits.
     (incomplete, return_position_impl_trait_in_trait, "1.65.0", Some(91611), None),
+    /// Allows bounding the return type of AFIT/RPITIT.
+    (incomplete, return_type_notation, "CURRENT_RUSTC_VERSION", Some(109417), None),
     /// Allows `extern "rust-cold"`.
     (active, rust_cold_cc, "1.63.0", Some(97544), None),
     /// Allows the use of SIMD types in functions declared in `extern` blocks.