ConfigManager: Convert GPUDeterminismMode into an enum class

Makes the values strongly-typed and gets more identifiers out of the
global namespace.

We are forced to use anything that is not "None" to mean none, because
X11 is garbage in that it has:

\#define None 0L

Because clearly no one else will ever want to use that identifier for
anything in their own code (and is why you should prefix literally
any and all preprocessor macros you expose to library users in public
headers).

Source/Core/Core/ConfigManager.h

@@ -37,6 +37,8 @@ namespace PowerPC
 enum class CPUCore;
 }  // namespace PowerPC
 
+struct BootParameters;
+
 // DSP Backend Types
 #define BACKEND_NULLSOUND _trans("No Audio Output")
 #define BACKEND_ALSA "ALSA"
@@ -47,17 +49,15 @@ enum class CPUCore;
 #define BACKEND_OPENSLES "OpenSLES"
 #define BACKEND_WASAPI "WASAPI (Exclusive Mode)"
 
-enum GPUDeterminismMode
+enum class GPUDeterminismMode
 {
-  GPU_DETERMINISM_AUTO,
-  GPU_DETERMINISM_NONE,
+  Auto,
+  Disabled,
   // This is currently the only mode.  There will probably be at least
   // one more at some point.
-  GPU_DETERMINISM_FAKE_COMPLETION,
+  FakeCompletion,
 };
 
-struct BootParameters;
-
 struct SConfig
 {
   // Wii Devices