Coherence does not properly handle VtableObject impls

[arielb1]

Directly having an impl Trait for Trait and cause a "coherence failed to report ambiguity" ICE. Having an indirect impl (impl<T: ?Sized> Trait for T) causes cryptic "type annotations required: cannot resolve Bar : Bar" and "recursion limit exceeded during monomorphisation" errors, but I didn't manage to get an ICE.

An example that causes a coherence failed to report ambiguity ICE:

trait Bar { fn bar(&self); }

impl Bar for Bar {
    fn bar(&self) { println!("trait-object impl called"); }
}

impl Bar for () {
    fn bar(&self) { println!("unit impl called"); }
}

fn main() {
}

[arielb1]

This is kind-of related to #18737, but not really (that one had a confusing issue with <Box<Any> as Any>, but this is a problem with <Any as Any>)

[arielb1]

This is fixed.