Replace no_ord_impl with orderable.

Similar to the previous commit, this replaces `newtype_index`'s opt-out
`no_ord_impl` attribute with the opt-in `orderable` attribute.

compiler/rustc_data_structures/src/graph/dominators/mod.rs

@@ -23,6 +23,7 @@ struct PreOrderFrame<Iter> {
 }
 
 rustc_index::newtype_index! {
+    #[orderable]
     struct PreorderIndex {}
 }