[lldb] [llgs] Refactor SendStopReasonForState for multiprocess

Refactor GDBRemoteCommunicationServerLLGS::SendStopReasonForState()
to accept process as an argument rather than hardcoding
m_current_process, in order to make it work correctly for multiprocess
scenarios.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127497