Fix a deadlock when connecting to a remote GDB server that might not support...

Fix a deadlock when connecting to a remote GDB server that might not support all packets that lldb-server or debugserver supports. The issue was the m_last_stop_packet_mutex mutex was being held by another thread and it was deadlocking getting the thread list. We now try to lock the m_last_stop_packet_mutex, and only continue if we successfully lock it. Else we fall back to qfThreadInfo/qsThreadInfo.

<rdar://problem/22140023>

llvm-svn: 252005