don't ice on non-lifetime binders under `-Zassumptions-on-binders` fixes rust-lang/rust#157778 w/ `-Znext-solver=globally -Zassumptions-on-binders` and a non-lifetime binder we hit `assert!(max_universe < u)` in `pull_region_outlives_constraints_out_of_universe` and ICE. the machinery here is region-outlives only, but a `for<T>` binder leaves a placeholder type in universe `u` that the region-only rewrite can't pull out, so an alias outlives like `<!T as Trait>::Assoc: 'r` reaches that assert still in `u`. just report ambiguity in that case, same as the `None => Ambiguity` bails right next to it — now it errors normally (E0284) instead of ICEing. r? @lcnr