Auto merge of #123737 - compiler-errors:alias-wf, r=lcnr

Check alias args for WF even if they have escaping bound vars

#### What

This PR stops skipping arguments of aliases if they have escaping bound vars, instead recursing into them and only discarding the resulting obligations referencing bounds vars.

#### An example:

From the test:
```
trait Trait {
    type Gat<U: ?Sized>;
}

fn test<T>(f: for<'a> fn(<&'a T as Trait>::Gat<&'a [str]>)) where for<'a> &'a T: Trait {}
//~^ ERROR the size for values of type `[()]` cannot be known at compilation time

fn main() {}
```

We now prove that `str: Sized` in order for `&'a [str]` to be well-formed. We were previously unconditionally skipping over `&'a [str]` as it referenced a buond variable. We now recurse into it and instead only discard the `[str]: 'a` obligation because of the escaping bound vars.

#### Why?

This is a change that improves consistency about proving well-formedness earlier in the pipeline, which is necessary for future work on where-bounds in binders and correctly handling higher-ranked implied bounds. I don't expect this to fix any unsoundness.

#### What doesn't it fix?

Specifically, this doesn't check projection predicates' components are well-formed, because there are too many regressions: rust-lang/rust#123737 (comment)

Author: bors