Depsgraph: Add utility function for transform dependency

This is what modifiers are to use to indicate that they depend
on a transformation of the object itself.

Currently should be no functional changes, but in the future
this will allow to easily change transform operation depending
on whether there is a simulation associated with the object.

source/blender/modifiers/intern/MOD_datatransfer.c

@@ -128,7 +128,7 @@ static void updateDepsgraph(ModifierData *md, const ModifierUpdateDepsgraphConte
 
 		if (dtmd->flags & MOD_DATATRANSFER_OBSRC_TRANSFORM) {
 			DEG_add_object_relation(ctx->node, dtmd->ob_source, DEG_OB_COMP_TRANSFORM, "DataTransfer Modifier");
-			DEG_add_object_relation(ctx->node, ctx->object, DEG_OB_COMP_TRANSFORM, "DataTransfer Modifier");
+			DEG_add_modifier_to_transform_relation(ctx->node, "DataTransfer Modifier");
 		}
 	}
 }