[Reproducer] Move GDB Remote Packet into Utility. (NFC)

To support dumping the reproducer's GDB remote packets, we need the
(de)serialization logic to live in Utility rather than the GDB remote
plugin. This patch renames StreamGDBRemote to GDBRemote and moves the
relevant packet code there.

Its uses in the GDBRemoteCommunicationHistory and the
GDBRemoteCommunicationReplayServer are updated as well.

Differential revision: https://reviews.llvm.org/D67523

llvm-svn: 371907