internal: Migrate assists to the structured snippet API, part 3
Continuing from #15231
Migrates the following assists:
- `add_missing_match_arms`
- `fix_visibility`
- `promote_local_to_const`
The `add_missing_match_arms` changes are best reviewed commit-by-commit since they're relatively big changes compared to the rest of the commits.
f45fdd9123