Add incomplete feature gate for inherent associate types.

[PoignardAzur]

Mentored by @oli-obk

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

struct Foo;

impl Foo {
    type Bar = isize;
}

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

let x : Foo::Bar;

will give you:

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`

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[rust-highfive]

r? @davidtwco

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[oli-obk]

r? @oli-obk

[oli-obk]

@bors r+

[bors]

📌 Commit 4f4e15d has been approved by oli-obk

[bors]

⌛ Testing commit 4f4e15d with merge 30e2baab56af30b9bd3c453fbc5e209f35ca68d1...

[bors]

💥 Test timed out

[rust-log-analyzer]

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[oli-obk]

@bors retry