Fix rare failure in TestProcessIO

[labath]

Summary

There was a race condition in Process class, where we would not wait for process stdout to
propagate fully before we would shut down the connection (repro case: slow down the stdio thread
by placing a sleep right at the end of the while loop in Communication::ReadThread). The Process
class already tried to solve this problem by synchronizing with the read thread in
Process::ShouldBroadcastEvent, but unfortunately the connection got closed before that in
Process::SetExitStatus. I solve this issue by delaying the connection shutdown until we get a
chance to process the event and synchronize. Alternatively, I could have moved the
synchronization point to an earlier point in SetExitStatus, but it seems safer to delay the
shutdown until other things get a chance to notice the process has exited.

[clayborg]

Looks fine.

source/Target/Process.cpp

Process::SetExitStatus (int status, const char *cstr)
     }
 
     m_exit_status = status;
     if (cstr)
         m_exit_string = cstr;
     else
         m_exit_string.clear();
 
-    // When we exit, we no longer need to the communication channel
-    m_stdio_communication.Disconnect();
-    m_stdio_communication.StopReadThread();
-    m_stdin_forward = false;
-
-    // And we don't need the input reader anymore as well
+    // When we exit, we don't need the input reader anymore
     if (m_process_input_reader)
     {
         m_process_input_reader->SetIsDone(true);
         m_process_input_reader->Cancel();
         m_process_input_reader.reset();
     }
 
     // Clear the last natural stop ID since it has a strong
Process::ShouldBroadcastEvent (Event *event_ptr)
     Log *log(lldb_private::GetLogIfAnyCategoriesSet(LIBLLDB_LOG_EVENTS | LIBLLDB_LOG_PROCESS));
     
     switch (state)
     {
         case eStateDetached:
         case eStateExited:
         case eStateUnloaded:
             m_stdio_communication.SynchronizeWithReadThread();
+            m_stdio_communication.Disconnect();
+            m_stdio_communication.StopReadThread();
+            m_stdin_forward = false;
+
             // fall-through
         case eStateConnected:
         case eStateAttaching:
         case eStateLaunching:
             // These events indicate changes in the state of the debugging session, always report them.
             return_value = true;
             break;
         case eStateInvalid: