Auto merge of #101682 - compiler-errors:rpitit-encode, r=fee1-dead

Only encode return-position `impl Trait` in trait when parent function has a default body

Semi-blocked on #101679, because I can't currently write a test for when we _should_ encode the type of the return-position `impl Trait` in trait, which is when a trait has a default function body, like so:

```rust
trait Foo {
  fn bar() -> impl Sized { }
}
```

Though this can land even without #101679, since it does prevent ICEs from occuring any time you use `#![feature(return_position_impl_trait_in_trait)]` in a library, which is kind annoying.

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