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_ast/src/ast.rs

@@ -2574,6 +2574,7 @@ pub enum AttrStyle {
 }
 
 rustc_index::newtype_index! {
+    #[orderable]
     #[debug_format = "AttrId({})"]
     pub struct AttrId {}
 }