Handle O reply packets during qRcmd

Summary:
Gdb servers like openocd may send many $O reply packets for the client to output during a qRcmd command sequence.  Currently, lldb interprets the first O packet as an unexpected response.  Besides generating no output, this causes lldb to get out of sync with future commands because it continues reading O packets from the first command as response to subsequent commands.

This patch handles any O packets during an qRcmd, treating the first non-O packet as the true response.

Preliminary discussion at http://lists.llvm.org/pipermail/lldb-dev/2018-January/013078.html

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D41745
Patch by Owen Shaw <llvm@owenpshaw.net>

llvm-svn: 322190