Added new options to lldb-platform:
    --gdbserver-port PORT
    --min-gdbserver-port PORT
    --max-gdbserver-port PORT

The --gdbserver-port option can be specified multiple times to tell lldb-platform which ports it can use when launching child GDB server processes.
The --min-gdbserver-port and --max-gdbserver-port options allow a range of ports to be specified for use when launching child GDB server processes.

Fixed the code to manage these ports correctly in GDBRemoteCommunicationServer.

Also changed GDBRemoteCommunicationClient to not set a port when sending the "qLaunchGDBServer" packet so that the remote lldb-platform can decide which ports to use. If the lldb-platform was launched with no --gdbserver-port or --min-gdbserver-port/--max-gdbserver-port options, then port 0 is always used and a unix socket is used between the lldb-platform and child GDB server process to coordinate the use of valid port.