cc: fix kw.rs warnings — use anonymous entries for tag-only keywords

Use `_` as the name for keyword entries that are only needed for interning
and tag-based membership (nullability qualifiers, attribute names, fortified
builtins, etc.). The define_ids! macro skips pub const emission for `_`
entries, eliminating 71 dead_code warnings without #[allow(dead_code)].

Also fix redundant_closure clippy lint in is_nullability_qualifier call,
and remove unused tags() function.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jgarzik