Update a debug string

Author: oli-obk

src/librustc_incremental/persist/dirty_clean.rs

@@ -394,7 +394,7 @@ impl<'a, 'tcx> DirtyCleanVisitor<'a, 'tcx> {
                     //HirItem::Trait(..) => ("ItemTrait", LABELS_TRAIT),
 
                     // An implementation, eg `impl<A> Trait for Foo { .. }`
-                    HirItem::Impl(..) => ("ItemImpl", LABELS_IMPL),
+                    HirItem::Impl(..) => ("ItemKind::Impl", LABELS_IMPL),
 
                     _ => self.tcx.sess.span_fatal(
                         attr.span,