regionck uses wrong body-id, overlooks some region obligations

[nikomatsakis]

While working on #41327, I encountered this bug. The following program is accepted but should not be:

trait Foo {
    fn foo<'a, T: Iterator>(&self) where T: 'a;
}

impl Foo for () {
    fn foo<'a, T: Iterator>(&self) where T::Item: 'a {
    }
}

fn main() { }

The fix is fairly straightforward: compare_method needs to use the impl-method id for the "body_id" field, not the id of the impl method's body. This is because it calls regionck_item which assumes it will do so. I'll have a PR up shortly.

[nikomatsakis]

triage: P-high

[nikomatsakis]

Oh, wait, maybe this should work =) -- in particular, since we know that T: 'a, we also know that T::Item: 'a.

I still feel there's a bug here, but this doesn't demonstrate it...