Use UnordSet instead of FxHashSet in define_id_collections!().

clippy_lints/src/len_zero.rs

@@ -219,7 +219,7 @@ fn fill_trait_set(traitt: DefId, set: &mut DefIdSet, cx: &LateContext<'_>) {
         let is_empty = sym!(is_empty);
 
         let is_empty_method_found = current_and_super_traits
-            .iter()
+            .items()
             .flat_map(|&i| cx.tcx.associated_items(i).filter_by_name_unhygienic(is_empty))
             .any(|i| {
                 i.kind == ty::AssocKind::Fn