The current linux implementation has a 1 second timeout when polling for
memory pressure. lldb-dap may take up to an extra 1 second to shutdown.
Use an event file descriptor to immediately stop the memory monitor
thread.
Fixes#150220
This implements a memory monitor for macOS, Linux and Windows. It
registers a callback that invokes `SBDebugger::MemoryPressureDetected`
when a low memory event is detected. This is motivated by the new
server mode, where the lldb-dap process will live across multiple
debug sessions and will use more memory due to caching.