return type sized check occurs in opaque defining env

[compiler-errors]

tests/ui/generic-associated-types/issue-92033.rs

fn foo() -> impl ?Sized {
    //~^ ERROR the size for values of type `impl ?Sized` cannot be known at compilation time
    ()
}

...passes in new solver because the sized check happens during typeck, and that allows us to reveal impl ?Sized: Sized to (): Sized. Not sure if that's desirable; if it's not, then we should probably move the return type sized check to wfcheck.

I think it's fine that this passes, tho.