Some destructor/drop related tweaks Two random tweaks I got from investigating some stuff around drops in edition 2024: 1. Use the `TypingEnv` of the mir builder, rather than making it over again. 2. Rename the `id` field from `Scope` to `local_id`, to reflect that it's a local id, and remove the `item_local_id()` accessor which just returned the id field.