[lldb] Adjust MainLoop on Win32 to work with mingw (llvm#163419)

This adjusts the monitor tear down to only call CancelIoEx a single
time, which should cancel all IO operations in the monitoring thread,
instead of calling it in a loop.

This removes the `m_monitor_thread.native_handle()` call that returns a
different type on windows/mingw compiles with pthreads enabled and
should fix llvm#162801.

Author: ashgti

lldb/source/Host/windows/MainLoopWindows.cpp

@@ -55,11 +55,7 @@ class PipeEvent : public MainLoopWindows::IOEvent {
     if (m_monitor_thread.joinable()) {
       m_stopped = true;
       SetEvent(m_ready);
-      // Keep trying to cancel ReadFile() until the thread exits.
-      do {
-        CancelIoEx(m_handle, /*lpOverlapped=*/NULL);
-      } while (WaitForSingleObject(m_monitor_thread.native_handle(), 1) ==
-               WAIT_TIMEOUT);
+      CancelIoEx(m_handle, /*lpOverlapped=*/NULL);
       m_monitor_thread.join();
     }
     CloseHandle(m_event);