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_index/src/vec/tests.rs

@@ -2,6 +2,7 @@
 use crate as rustc_index;
 
 crate::newtype_index! {
+    #[orderable]
     #[max = 0xFFFF_FFFA]
     struct MyIdx {}
 }