Rollup merge of #82516 - PoignardAzur:inherent-impl-ty, r=oli-obk

Add incomplete feature gate for inherent associate types.

Mentored by ``````@oli-obk``````

So far the only change is that instead of giving an automatic error, the following code compiles:

```rust
struct Foo;

impl Foo {
    type Bar = isize;
}
```

The backend work to make it actually usable isn't there yet. In particular, this:

```rust
let x : Foo::Bar;
```

will give you:

```sh
error[E0223]: ambiguous associated type
  --> /$RUSTC_DIR/src/test/ui/assoc-inherent.rs:15:13
   |
LL |     let x : Foo::Bar;
   |             ^^^^^^^^ help: use fully-qualified syntax: `<Foo as Trait>::Bar`
```

compiler/rustc_span/src/symbol.rs

@@ -634,6 +634,7 @@ symbols! {
         index_mut,
         infer_outlives_requirements,
         infer_static_outlives_requirements,
+        inherent_associated_types,
         inlateout,
         inline,
         inline_const,