Make #[debug_format] an attribute in newtype_index

This removes the `custom` format functionality as its only user was
trivially migrated to using a normal format.

If a new use case for a custom formatting impl pops up, you can add it
back.

compiler/rustc_ast/src/ast.rs

@@ -2555,8 +2555,8 @@ pub enum AttrStyle {
 
 rustc_index::newtype_index! {
     #[custom_encodable]
+    #[debug_format = "AttrId({})]"]
     pub struct AttrId {
-        DEBUG_FORMAT = "AttrId({})"
     }
 }
 

compiler/rustc_ast/src/node_id.rs

@@ -8,8 +8,8 @@ rustc_index::newtype_index! {
     /// This is later turned into [`DefId`] and `HirId` for the HIR.
     ///
     /// [`DefId`]: rustc_span::def_id::DefId
+    #[debug_format = "NodeId({})"]
     pub struct NodeId {
-        DEBUG_FORMAT = "NodeId({})"
     }
 }