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`
```
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