Auto merge of #74670 - tmandry:issue-73818, r=matthewjasper

Normalize bounds fully when checking defaulted types

When checking that the default type for `<T as X>::Y` is valid in this example:

```
trait X { type Y: PartialEq<<Self as X>::Y> }
impl X for T { default type Y = S; }
```

We will have to prove the bound `S: PartialEq<<T as X>::Y>`. In this case
we want `<T as X>::Y` to normalize to `S`. This is valid because we are
checking the default value specifically here. Add `<T as X>::Y = S` to the
ParamEnv for normalization _of the bound we are checking_ only.

Fixes #73818.

---

I noticed that adding this to the env for bounds checking didn't break any tests. Not sure if this is because we can't rely on it to prove anything, or because of missing test coverage.

r? @matthewjasper, @nikomatsakis

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