Support ?Trait bounds in supertraits and dyn Trait under a feature gate

compiler/rustc_hir/src/intravisit.rs

@@ -902,7 +902,9 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) -> V::Resul
             try_visit!(visitor.visit_array_length(length));
         }
         TyKind::TraitObject(bounds, ref lifetime, _syntax) => {
-            walk_list!(visitor, visit_poly_trait_ref, bounds);
+            for (bound, _modifier) in bounds {
+                try_visit!(visitor.visit_poly_trait_ref(bound));
+            }
             try_visit!(visitor.visit_lifetime(lifetime));
         }
         TyKind::Typeof(ref expression) => try_visit!(visitor.visit_anon_const(expression)),