Eliminate DefiningAnchor now that is just a single-variant enum

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -316,6 +316,6 @@ fn response_no_constraints_raw<'tcx>(
             external_constraints: tcx.mk_external_constraints(ExternalConstraintsData::default()),
             certainty,
         },
-        defining_anchor: Default::default(),
+        defining_opaque_types: Default::default(),
     }
 }