Use a stub AchivementManager when USE_RETRO_ACHIEVEMENTS isn't defined

This lets us reduce the number of USE_RETRO_ACHIEVEMENTS ifdefs in the
code base, reducing visual clutter. In particular, needing an ifdef for
each call to IsHardcodeModeActive was annoying to me. This also reduces
the risk that someone writes code that accidentally fails to compile
with USE_RETRO_ACHIEVEMENTS disabled.

We could cut down on ifdefs even further by making HardcodeWarningWidget
always exist, but that would result in non-trivial code ending up in the
binary even with USE_RETRO_ACHIEVEMENTS disabled, so I'm leaving it out
of this PR. It's not a lot of code though, so I might end up revisiting
it at some point.

Source/Core/Core/AchievementManager.h

@@ -5,19 +5,25 @@
 
 #ifdef USE_RETRO_ACHIEVEMENTS
 #include <array>
+#include <chrono>
 #include <ctime>
 #include <functional>
 #include <map>
+#include <memory>
 #include <mutex>
+#include <set>
 #include <string>
+#include <string_view>
 #include <thread>
 #include <unordered_map>
+#include <vector>
 
 #include <rcheevos/include/rc_api_runtime.h>
 #include <rcheevos/include/rc_api_user.h>
 #include <rcheevos/include/rc_client.h>
 #include <rcheevos/include/rc_runtime.h>
 
+#include "Common/CommonTypes.h"
 #include "Common/Event.h"
 #include "Common/HttpRequest.h"
 #include "Common/WorkQueueThread.h"
@@ -206,4 +212,31 @@ private:
   std::recursive_mutex m_filereader_lock;
 };  // class AchievementManager
 
+#else  // USE_RETRO_ACHIEVEMENTS
+
+#include <string>
+
+namespace DiscIO
+{
+class Volume;
+}
+
+class AchievementManager
+{
+public:
+  static AchievementManager& GetInstance()
+  {
+    static AchievementManager s_instance;
+    return s_instance;
+  }
+
+  constexpr bool IsHardcoreModeActive() { return false; }
+
+  constexpr void LoadGame(const std::string&, const DiscIO::Volume*) {}
+
+  constexpr void DoFrame() {}
+
+  constexpr void CloseGame() {}
+};
+
 #endif  // USE_RETRO_ACHIEVEMENTS