Make DiscIO/ mostly IWYU clean (and fix errors in rest of the project detected by this change).

Source/Core/DiscIO/BannerLoader.h

@@ -7,10 +7,14 @@
 #include <string>
 #include <vector>
 
-#include "DiscIO/Filesystem.h"
+#include "Common/CommonTypes.h"
 
 namespace DiscIO
 {
+
+class IFileSystem;
+class IVolume;
+
 class IBannerLoader
 {
 	public:
@@ -33,4 +37,5 @@ class IBannerLoader
 };
 
 IBannerLoader* CreateBannerLoader(DiscIO::IFileSystem& _rFileSystem, DiscIO::IVolume *pVolume);
-} // namespace
+
+}  // namespace DiscIO