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_borrowck/src/member_constraints.rs

@@ -56,8 +56,7 @@ pub(crate) struct NllMemberConstraint<'tcx> {
 
 rustc_index::newtype_index! {
     #[debug_format = "MemberConstraintIndex({})"]
-    pub(crate) struct NllMemberConstraintIndex {
-    }
+    pub(crate) struct NllMemberConstraintIndex {}
 }
 
 impl Default for MemberConstraintSet<'_, ty::RegionVid> {