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_trait_selection/src/solve/search_graph.rs

@@ -13,6 +13,7 @@ use rustc_session::Limit;
 use std::collections::hash_map::Entry;
 
 rustc_index::newtype_index! {
+    #[orderable]
     pub struct StackDepth {}
 }