Fix lock hierarchy violation in Listener/Broadcaster
- avoid deadlocks if Broadcaster::SignUpListenersForBroadcaster and
  Listener::StartListeningForEventSpec are both called concurrently



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180050 91177308-0d34-0410-b5e6-96231b3b80d8