Properly handle when commands are not unsupported in the GDB remote clients.

Prior to this fix we would often call SendPacketAndWaitForResponse() which
returns the number of bytes in the response. The UNSUPPORTED response in the
GDB remote protocol is zero bytes and we were checking for it inside an if
statement:

if (SendPacketAndWaitForResponse(...))
{
    if (response.IsUnsupportedResponse())
    {
        // UNSUPPORTED...
        // This will never happen...
    }
}

We now handle is properly as:

if (SendPacketAndWaitForResponse(...))
{
}
else
{
    // UNSUPPORTED...
}

llvm-svn: 131393