Auto merge of #3961 - RalfJung:event-release-clock-join, r=RalfJung

epoll event adding: no need to join, there's no old clock here

src/shims/unix/linux/epoll.rs

@@ -626,7 +626,7 @@ fn check_and_update_one_event_interest<'tcx>(
         let mut event_instance = EpollEventInstance::new(flags, epoll_event_interest.data);
         // If we are tracking data races, remember the current clock so we can sync with it later.
         ecx.release_clock(|clock| {
-            event_instance.clock.join(clock);
+            event_instance.clock.clone_from(clock);
         });
         // Triggers the notification by inserting it to the ready list.
         ready_list.insert(epoll_key, event_instance);