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_span/src/hygiene.rs

@@ -84,8 +84,8 @@ rustc_index::newtype_index! {
     /// A unique ID associated with a macro invocation and expansion.
     #[custom_encodable]
     #[no_ord_impl]
+    #[debug_format = "expn{}"]
     pub struct LocalExpnId {
-        DEBUG_FORMAT = "expn{}"
     }
 }