[lldb-dap] Implement a MemoryMonitor (#129332)
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.
