Do not register `Self: AutoTrait` when confirming auto trait (in old solver)
Every built-in auto impl for a trait goal like `Ty: Auto` immediately registers another obligation of `Ty: Auto` as one of its nested obligations, leading to us stressing the cycle detection machinery a lot more than we need to. This is because all traits have a `Self: Trait` predicate.
To fix this, remove the call to `impl_or_trait_obligations` in `vtable_auto_impl`, since auto traits do not have where clauses.
r? lcnr
961351c76c