When encountering an incorrect trait bound referencing the current trait, suggest correct code

[estebank]

The following snippet is not correct:

trait Bar {}

trait Foo where Foo::Assoc: Bar {
    type Assoc;
}

But these three are:

trait Foo where Self::Assoc: Bar {
    type Assoc;
}

trait Foo where <Self as Foo>::Assoc: Bar {
    type Assoc;
}

trait Foo {
    type Assoc: Bar;
}

For the first we currently emit:

error[E0223]: ambiguous associated type
 --> file.rs:3:17
  |
3 | trait Foo where Foo::Assoc: Bar {
  |                 ^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Foo>::Assoc`

but it should suggest one of the two valid snippets above.