unlike closure parent args, we check member constraints for unused early-bound `FnDef` lifetimes

[lcnr]

FnDef lifetime arguments are ignored by outlives constraints. They are not ignored by member constraints. This is inconsistent.

fn cool<'a: 'a>() {}

fn closure_outlives<'a>() -> impl Sized + use<'a> + 'static {
    || ()
}
fn fn_def_outlives<'a>() -> impl Sized + use<'a> + 'static {
    cool::<'a>
}

fn closure_member_constraints<'a>() -> impl Sized + use<> {
    || ()
}
fn fn_def_member_constraints<'a>() -> impl Sized + use<> {
    cool::<'a> //~ ERROR hidden type for `impl Sized` captures lifetime that does not appear in bounds
}

We shouldn't try to make this consistent before resolving #153140 (comment)