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

[compiler-errors]

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:

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.

[rust-highfive]

r? @fee1-dead

(rust-highfive has picked a reviewer for you, use r? to override)

[fee1-dead]

@bors r+ rollup

[bors]

📌 Commit 1335da9 has been approved by fee1-dead

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1335da9 with merge 17a627f...

[bors]

☀️ Test successful - checks-actions
Approved by: fee1-dead
Pushing 17a627f to master...