Common: Add header for compiler-specifics

Instead of globbing things under an ambiguous Common.h header, move
compiler-specifics over to Compiler.h. This gives us a dedicated home
for anything related to compilers that we want to make functional across
all compilers that we support.

This moves us a little closer to eliminating Common.h entirely.

Source/Core/VideoCommon/IndexGenerator.cpp

@@ -4,8 +4,8 @@
 
 #include <cstddef>
 
-#include "Common/Common.h"
 #include "Common/CommonTypes.h"
+#include "Common/Compiler.h"
 #include "Common/Logging/Log.h"
 #include "VideoCommon/IndexGenerator.h"
 #include "VideoCommon/OpcodeDecoding.h"