Associated type binding completions show up when they are not allowed

```rust
struct S;
trait Tr<T> {
    type Ty;
}

impl Tr<$0
```

This suggests using `Ty = ` to specify the associated type, but that's not allowed when implementing a trait, only when using `dyn Trait` or `impl Trait` as a type, and when using a trait in a where-clause (perhaps whether it's allowed should be stored in the context).