Correctly substitute GAT's type used in `normalize_param_env` in `check_type_bounds`
Given:
```rust
trait Foo {
type Assoc<T>: PartialEq<Self::Assoc<i32>>;
}
impl Foo for () {
type Assoc<T> = Wrapper<T>;
}
struct Wrapper<T>(T);
impl<T> PartialEq<Wrapper<i32>> for Wrapper<T> { }
```
We add an additional predicate in the `normalize_param_env` in `check_type_bounds` that is used to normalize the GAT's bounds to check them in the impl. Problematically, though, that predicate is constructed to be `for<^0> <() as Foo>::Assoc<^0> => Wrapper<T>`, instead of `for<^0> <() as Foo>::Assoc<^0> => Wrapper<^0>`.
That means `Self::Assoc<i32>` in the bounds that we're checking normalizes to `Wrapper<T>`, instead of `Wrapper<i32>`, and so the bound `Self::Assoc<T>: PartialEq<Self::Assoc<i32>>` normalizes to `Wrapper<T>: PartialEq<Wrapper<T>>`, which does not hold.
Fixes this by properly substituting the RHS of that normalizes predicate that we add to the `normalize_param_env`. That means the bound is properly normalized to `Wrapper<T>: PartialEq<Wrapper<i32>>`, which *does* hold.
---
The second commit in this PR just cleans up some substs stuff and some naming.
r? `@jackh726` cc #87900
7e90732abe