Update config selector for LDS update with no route config (#25528)

* Update config selector upon LDS update

* Add current_virtual_host_.Clear()

* Correct the clear statement

src/core/ext/filters/client_channel/resolver/xds/xds_resolver.cc

@@ -732,6 +732,7 @@ void XdsResolver::OnListenerUpdate(XdsApi::LdsUpdate listener) {
     }
     route_config_name_ = std::move(listener.route_config_name);
     if (!route_config_name_.empty()) {
+      current_virtual_host_.routes.clear();
       auto watcher = absl::make_unique<RouteConfigWatcher>(Ref());
       route_config_watcher_ = watcher.get();
       xds_client_->WatchRouteConfigData(route_config_name_, std::move(watcher));
@@ -741,6 +742,10 @@ void XdsResolver::OnListenerUpdate(XdsApi::LdsUpdate listener) {
   if (route_config_name_.empty()) {
     GPR_ASSERT(current_listener_.rds_update.has_value());
     OnRouteConfigUpdate(std::move(*current_listener_.rds_update));
+  } else {
+    // HCM may contain newer filter config. We need to propagate the update as
+    // config selector to the channel
+    GenerateResult();
   }
 }