no normalization of unused alias definitions in defaulted type parameters

We don't eagerly try to normalize defaulted type parameters, allowing the default to use a diverging alias.

This example was mentioned by @BoxyUwU within the context of the new solver, in https://github.com/rust-lang/trait-system-refactor-initiative/issues/139#issuecomment-2704576249, but this equally applies to both solvers. Therefore, we can track this in an issue here instead.

#![feature(rustc_attrs)]
#![rustc_no_implicit_bounds]

trait Trait {
    type Diverges<D: Trait>;
}

impl<T> Trait for T {
    type Diverges<D: Trait> = D::Diverges<D>;
}

struct Bar<T = <u8 as Trait>::Diverges<u8>>(*mut T);

passes in both solvers.

Start with the Rust reproducer in the issue and the linked trait-system discussion, then trace how defaulted type parameters are handled by both solvers. Done means the diverging alias is normalized consistently and regression coverage verifies the behavior in both solvers.