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_middle/src/mir/coverage.rs

@@ -18,6 +18,7 @@ rustc_index::newtype_index! {
     /// to use a larger representation on the Rust side.
     #[derive(HashStable)]
     #[encodable]
+    #[orderable]
     #[max = 0xFFFF_FFFF]
     #[debug_format = "CounterId({})"]
     pub struct CounterId {}
@@ -39,6 +40,7 @@ rustc_index::newtype_index! {
     /// to use a larger representation on the Rust side.
     #[derive(HashStable)]
     #[encodable]
+    #[orderable]
     #[max = 0xFFFF_FFFF]
     #[debug_format = "ExpressionId({})"]
     pub struct ExpressionId {}