Fixed the GDBRemoteCommuncation to return a new GDBRemoteCommuncation::PacketResult enum for all packet sends/receives.

<rdar://problem/15600045>

Due to other recent changes, all connections to GDB servers that didn't support the "QStartNoAckMode" packet would cause us to fail to attach to the remote GDB server.

The problem was that SendPacket* and WaitForResponse* packets would return a size_t indicating the number of bytes sent/received. The other issue was WaitForResponse* packets would strip the leading '$' and the trailing "#CC" (checksum) bytes, so the unimplemented response packet of "$#00" would get stripped and the WaitForResponse* packets would return 0.

These new error codes give us flexibility to to more intelligent things in response to what is returned.