alias bound candidates for normalizable aliases allows more code to compile

[lcnr]

The new solver uses alias bound candidates for aliases which previously were eagerly normalized, causing in the following example to now compile: https://rust.godbolt.org/z/E3e6WqoEb

trait Bound {}

trait Trait {
    type Assoc: Bound; 
}

fn impls_bound<T: Bound>() {}

fn foo<T: Trait<Assoc = U>, U>() {
    impls_bound::<<T as Trait>::Assoc>();
}