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_transform/src/coverage/graph.rs

@@ -264,6 +264,7 @@ impl graph::WithPredecessors for CoverageGraph {
 
 rustc_index::newtype_index! {
     /// A node in the control-flow graph of CoverageGraph.
+    #[orderable]
     #[debug_format = "bcb{}"]
     pub(super) struct BasicCoverageBlock {
         const START_BCB = 0;