llgs: implement qThreadStopInfo.

This change implements this ticket:
http://llvm.org/bugs/show_bug.cgi?id=20899

Adds the qThreadStopInfo RSP command for llgs and includes a test that
verifies both debugserver and llgs respond with something reasonable
on a multithreaded app.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217549 91177308-0d34-0410-b5e6-96231b3b80d8