Rollup merge of #107758 - Zoxc:arena-enref, r=cjgillot

Change `arena_cache` to not alter the declared query result

This makes the return types a bit clearer, limiting `arena_cache`'s effect to just the computation side. It also makes it easier to potentially remove `arena_cache`.

r? ``@cjgillot``

Author: matthiaskrgr