[lldb] Improve assert in GDBRemoteCommunicationReplayServer

While investigating an issue where a different packet was sent during
replay I noticed how annoying it is that the existing assert doesn't
specify what packet is actually different. It's printed to the log, but
enabling logging has the potential to change LLDB's behavior. The same
is true when debugging LLDB while it's replaying the reproducer.

I replaced the assert with a printf of the unexpected packet followed by
a fatal_error wrapped in ifndef NDEBUG. The behavior is the same as the
previous assert, just with more/better context.