ICE when default method relies on where clauses that object type does not satisfy

[nikomatsakis]

This is problem blocking #23300:

struct Foo {
    x: i32
}

trait Bar {
    fn bar(&self) where Self : Baz { self.baz(); }
}

trait Baz {
    fn baz(&self);
}

impl Bar for Foo {
}

fn main() {
    let x: &Bar = &Foo { x: 22 };
}

That example encounters an ICE when building the vtable of Bar for Foo. The problem is that the default method bar requires that Self:Baz, which is not true for Foo. This is ok, since it could never be called, but the trans code doesn't know that (yet!).

[nikomatsakis]

Got a fix that seems to be working. Running make check now.

[nikomatsakis]

(Branch is issue-23435-default-methods-with-where-clauses on my repo)