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_lint/src/levels.rs

@@ -41,7 +41,7 @@ struct LintLevelSets {
 rustc_index::newtype_index! {
     #[custom_encodable] // we don't need encoding
     struct LintStackIndex {
-        const COMMAND_LINE = 0,
+        const COMMAND_LINE = 0;
     }
 }