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_mir_dataflow/src/move_paths/mod.rs

@@ -14,6 +14,7 @@ use self::abs_domain::{AbstractElem, Lift};
 mod abs_domain;
 
 rustc_index::newtype_index! {
+    #[orderable]
     #[debug_format = "mp{}"]
     pub struct MovePathIndex {}
 }
@@ -25,6 +26,7 @@ impl polonius_engine::Atom for MovePathIndex {
 }
 
 rustc_index::newtype_index! {
+    #[orderable]
     #[debug_format = "mo{}"]
     pub struct MoveOutIndex {}
 }