opaque type auto trait leakage is used even when item bounds would be sufficient

[lcnr]

fn foo() -> impl Send {
    bar()
}

fn is_send<T: Send>(_: T) {} 

fn bar() -> impl Send {
    if false {
        is_send(foo()); // failure in new solver only
    }
}

also affects the following RPITIT test:

• tests/ui/impl-trait/in-trait/refine-cycle.rs

We could stop auto trait leakage if the auto trait is part of the item bounds of the opaque. Lets wait until we encounter this issue in practice, even if I consider that change desirable regardless