Normalize the RHS of an Unsize goal in the new solver - #113393

Unsize goals are... tricky. Not only do they structurally match on their self type, but they're also structural on their other type parameter. I'm pretty certain that it is both incomplete and also just plain undesirable to not consider normalizing the RHS of an unsize goal. More practically, I'd like for this code to work:

trait A {}
trait B: A {}

impl A for usize {}
impl B for usize {}

trait Mirror {
    type Assoc: ?Sized;
}

impl<T: ?Sized> Mirror for T {
    type Assoc = T;
}

fn main() {
    // usize: Unsize<dyn B>
    let x = Box::new(1usize) as Box<<dyn B as Mirror>::Assoc>;
    // dyn A: Unsize<dyn B>
    let y = x as Box<<dyn A as Mirror>::Assoc>;
}

In order to achieve this, we add EvalCtxt::normalize_non_self_ty (naming modulo bikeshedding), which must be used for all non-self type arguments that are structurally matched in candidate assembly. Currently this is only necessary for Unsize's argument, but I could see future traits requiring this (hopefully rarely) in the future. It uses repeat_while_none to limit infinite looping, and normalizes the self type until it is no longer an alias.

Also, we need to fix feature gate detection for trait_upcasting and unsized_tuple_coercion when HIR typeck has unnormalized types. We can do that by checking the ImplSource returned by selection, which necessitates adding a new impl source for tuple upcasting.