ControllerInterface: Add platform consistent names for modifier keys.

Source/Core/InputCommon/ControllerInterface/DInput/DInputKeyboardMouse.cpp

@@ -87,6 +87,11 @@ KeyboardMouse::KeyboardMouse(const LPDIRECTINPUTDEVICE8 kb_device,
   for (u8 i = 0; i < sizeof(named_keys) / sizeof(*named_keys); ++i)
     AddInput(new Key(i, m_state_in.keyboard[named_keys[i].code]));
 
+  // Add combined left/right modifiers with consistent naming across platforms.
+  AddCombinedInput("Alt", {"LMENU", "RMENU"});
+  AddCombinedInput("Shift", {"LSHIFT", "RSHIFT"});
+  AddCombinedInput("Ctrl", {"LCONTROL", "RCONTROL"});
+
   // MOUSE
   DIDEVCAPS mouse_caps = {};
   mouse_caps.dwSize = sizeof(mouse_caps);