Make empty bounds lower to WellFormed and make WellFormed coinductive

compiler/rustc_privacy/src/lib.rs

@@ -145,6 +145,7 @@ where
                 }
                 ControlFlow::CONTINUE
             }
+            ty::PredicateKind::WellFormed(arg) => arg.visit_with(self),
             _ => bug!("unexpected predicate: {:?}", predicate),
         }
     }