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_ast/src/ast.rs

@@ -2556,8 +2556,7 @@ pub enum AttrStyle {
 rustc_index::newtype_index! {
     #[custom_encodable]
     #[debug_format = "AttrId({})]"]
-    pub struct AttrId {
-    }
+    pub struct AttrId {}
 }
 
 impl<S: Encoder> Encodable<S> for AttrId {