llgs-tests: Add support for "exit" stop-reply packets

Summary:
This makes StopReply class abstract, so that we can represent different
types of stop replies such as StopReplyStop and StopReplyExit (there
should also be a StopReplySignal, but I don't need that right now so I
haven't implemented it yet).

This prepares the ground for a new test I'm writing.

Reviewers: eugene, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D41067

llvm-svn: 320820