message: Add declarations for ObjC helper methods
diff: #define DECLARE_OBJC_HELPERS(CPPTYPE) \
  class WrappedMTL##CPPTYPE; \
  + extern WrappedMTL##CPPTYPE *GetWrapped(MTL::CPPTYPE *objCWrapped); \
  + extern MTL::CPPTYPE *GetReal(MTL::CPPTYPE *objCWrapped); \
  + extern bool IsObjCWrapped(MTL::CPPTYPE *objCWrapped); \
  + extern ResourceId GetId(MTL::CPPTYPE *objCWrapped); \
  extern MTL::CPPTYPE *AllocateObjCBridge(WrappedMTL##CPPTYPE *wrapped);
  METALCPP_WRAPPED_PROTOCOLS(DECLARE_OBJC_HELPERS)
  