[lldb] Fix ThreadList::Update not locking the rhs's mutex

[augusto2112]

Summary

ThreadList::Update is being caught by thread sanitizer. There's even a
comment on the function that both mutexes should be lock (even though
only the "this" mutex was actually being locked). Fix this by locking
both mutexes.

[JDevlieghere]

LGTM

lldb/source/Target/ThreadList.cpp

selected_thread_sp->BroadcastEvent(
         Thread::eBroadcastBitThreadSelected,
         new Thread::ThreadEventData(selected_thread_sp));
 }
 
 void ThreadList::Update(ThreadList &rhs) {
   if (this != &rhs) {
     // Lock both mutexes to make sure neither side changes anyone on us while
     // the assignment occurs
-    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    std::scoped_lock guard(GetMutex(), rhs.GetMutex());
 
     m_process = rhs.m_process;
     m_stop_id = rhs.m_stop_id;
     m_threads.swap(rhs.m_threads);
     m_selected_tid = rhs.m_selected_tid;
 
     // Now we look for threads that we are done with and make sure to clear
     // them up as much as possible so anyone with a shared pointer will still