Rollup merge of #135911 - Zalathar:arena-cache-option, r=compiler-errors

Allow `arena_cache` queries to return `Option<&'tcx T>`

Currently, `arena_cache` queries always have to return `&'tcx T`[^deref]. This means that if an arena-cached query wants to return an optional value, it has to return `&'tcx Option<T>`, which has a few negative consequences:

- It goes against normal Rust style, where `Option<&T>` is preferred over `&Option<T>`.
- Callers that actually want an `Option<&T>` have to manually call `.as_ref()` on the query result.
- When the query result is `None`, a full-sized `Option<T>` still needs to be stored in the arena.

This PR solves that problem by introducing a helper trait `ArenaCached` that is implemented for both `&T` and `Option<&T>`, and takes care of bridging between the provided type, the arena-allocated type, and the declared query return type.

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To demonstrate that this works, I have converted the two existing arena-cached queries that currently return `&Option<T>`: `mir_coroutine_witnesses` and `diagnostic_hir_wf_check`. Only the query declarations need to be modified; existing providers and callers continue to work with the new query return type.

(My real goal is to apply this to `coverage_ids_info`, which will return Option as of #135873, but that PR hasn't landed yet.)

[^deref]: Technically they could return other types that implement `Deref`, but it's hard to imagine this working well with anything other than `&T`.

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