A few small cleanups for newtype_index

Remove the `..` from the body, only a few invocations used it and it's
inconsistent with rust syntax.

Use `;` instead of `,` between consts. As the Rust syntax gods inteded.

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -284,7 +284,7 @@ rustc_index::newtype_index! {
     /// A node in the control-flow graph of CoverageGraph.
     #[debug_format = "bcb{}"]
     pub(super) struct BasicCoverageBlock {
-        const START_BCB = 0,
+        const START_BCB = 0;
     }
 }