drop_tracking_mir: support new solver.

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -149,6 +149,6 @@ fn drain_unstalled_obligations(
         &mut self,
         _: &InferCtxt<'tcx>,
     ) -> Vec<PredicateObligation<'tcx>> {
-        unimplemented!()
+        std::mem::take(&mut self.obligations)
     }
 }