Folding anon consts in `param_env` query can lead to ambiguity

```rust
trait Trait: Deref<Target = [u8; { 1 + 1 }]> {}
```

When elaborating param env for `Trait`, we have `Self: Trait`, `Self: Deref<Target = [u8; {anon const}]>`. We fold anon consts away *before* elaborating. However, we end up getting another *un-folded* copy of the anon const from elaborating `Self: Trait`. This leads to ambiguity.