Rollup merge of #110237 - oli-obk:impl_trait_in_assoc_tys, r=jackh726

Split out a separate feature gate for impl trait in associated types

in https://github.com/rust-lang/rust/issues/107645 it was decided that we'll take a new route for type alias impl trait. The exact route isn't clear yet, so while I'm working on implementing some of these proposed changes (e.g. in https://github.com/rust-lang/rust/pull/110010) to be able to experiment with them, I will also work on stabilizing another sugar version first: impl trait in associated types. Similarly I'll look into creating feature gates for impl trait in const/static types.

This PR does nothing but split the feature gate, so that you need to enable a different feature gate for

```rust
impl Trait for Type {
    type Assoc = impl SomeTrait;
}
```

than what you need for `type Foo = impl SomeTrait;`

src/librustdoc/lib.rs

@@ -15,6 +15,7 @@
 #![feature(type_ascription)]
 #![feature(iter_intersperse)]
 #![feature(type_alias_impl_trait)]
+#![cfg_attr(not(bootstrap), feature(impl_trait_in_assoc_type))]
 #![recursion_limit = "256"]
 #![warn(rustc::internal)]
 #![allow(clippy::collapsible_if, clippy::collapsible_else_if)]