XBox One: Use latest C++

Source/diablo.h

@@ -127,8 +127,4 @@ extern bool DebugDisableNetworkTimeout;
  */
 extern GameLogicStep gGameLogicStep;
 
-#ifdef __UWP__
-void setOnInitialized(void (*)());
-#endif
-
 } // namespace devilution

uwp-project/src/SDL_winrt_main_NonXAML.cpp

@@ -3,7 +3,6 @@
 */
 
 #include <SDL.h>
-#include <diablo.h>
 #include <wrl.h>
 
 /* At least one file in any SDL/WinRT app appears to require compilation
@@ -60,6 +59,13 @@ void onInitialized()
 	Windows::UI::Core::SystemNavigationManager::GetForCurrentView()->BackRequested += ref new Windows::Foundation::EventHandler<Windows::UI::Core::BackRequestedEventArgs^>(OnBackRequested);
 }
 
+// We do not include the headers for the declarations below directly
+// because C++/CX is stuck on pre C++20 language support.
+namespace devilution {
+void setOnInitialized(void (*)());
+int DiabloMain(int argc, char **argv);
+}  // namespace devilution
+
 int CALLBACK WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
 {
 	devilution::setOnInitialized(&onInitialized);