[lldb] Guarded GDBRemoteCommunicationServerLLGS::m_saved_registers_map (#219431)
m_saved_registers_map and m_next_saved_registers_id were guarded by a
dedicated std::mutex. This replaces the manual locking logic with
`Guarded`.
This also fixes the missing lock in the Handle_QSaveRegisterState
assert which was previously a race condition.