Fix Debugger::HandleProcessEvent in case when ProcessIOHandler doesn't exist

[ki.stfu]

Summary

Previously the Debugger::HandleProcessEvent hid a top IOHandler if the
process's IOHandler was inactive and later refreshed it. Usually the
IOHandler.Refresh() prints the (lldb) prompt. The problem was in case of
iOS remote platform when trying to execute 'command source' command.
On this platform the process's IOHandler is empty, therefore the
Debugger::HandleProcessEvent hid a top IOHandler and later refreshed it.
So that the (lldb) prompt was printed with a program output in mixed
order:
was:

longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong
longlonglonglonglonglonglonglonglonglonglonglonglonglonglon(lldb)
glonglonglonglonglonglonglonglonglonglonglonglong string

now:

longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong
longlonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglonglong
longlonglonglonglonglonglonglonglong string

[clayborg]

Looks good.

include/lldb/Target/Process.h

protected:
     bool
     PushProcessIOHandler ();
     
     bool
     PopProcessIOHandler ();
     
     bool
     ProcessIOHandlerIsActive ();
-    
+    bool
+    ProcessIOHandlerExists () const
+    {
+        return static_cast<bool>(m_process_input_reader);
+    }
+    
     Error
     HaltForDestroyOrDetach(lldb::EventSP &exit_event_sp);
 
     bool
     StateChangedIsExternallyHijacked();
 
     void
     LoadOperatingSystemPlugin(bool flush);

source/Core/Debugger.cpp

if (!gui_enabled)
             Process::HandleProcessStateChangedEvent (event_sp, &output_stream, pop_process_io_handler);
         }
 
         if (output_stream.GetSize() || error_stream.GetSize())
         {
             StreamFileSP error_stream_sp (GetOutputFile());
             bool top_io_handler_hid = false;
 
-            if (process_sp->ProcessIOHandlerIsActive() == false)
+            if (process_sp->ProcessIOHandlerExists() && process_sp->ProcessIOHandlerIsActive() == false)
                 top_io_handler_hid = HideTopIOHandler();
 
             if (output_stream.GetSize())
             {
                 StreamFileSP output_stream_sp (GetOutputFile());
                 if (output_stream_sp)
                     output_stream_sp->Write (output_stream.GetData(), output_stream.GetSize());
             }