Add an always-ambiguous predicate to make sure that we don't accidentlally allow trait resolution to prove false things during coherence

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -717,6 +717,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
+                ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
             }
         })
     }