Compiler: Use [[maybe_unused]] in the Windows DOLPHIN_UNUSED macro

The required version of MSVC already supports [[maybe_unused]], so we
can utilize this here. When GCC 7 and clang 3.9 become hard
requirements, we can eliminate this macro entirely and replace it with
[[maybe_unused]].

Source/Core/Common/Compiler.h

@@ -4,12 +4,13 @@
 
 #pragma once
 
+// TODO: Replace this with [[maybe_unused]] directly when GCC 7 and clang 3.9
+//       are hard requirements.
 #if defined(__GNUC__) || __clang__
 // Disable "unused function" warnings for the ones manually marked as such.
 #define DOLPHIN_UNUSED __attribute__((unused))
 #else
-// Not sure MSVC even checks this...
-#define DOLPHIN_UNUSED
+#define DOLPHIN_UNUSED [[maybe_unused]]
 #endif
 
 #ifdef _WIN32