8901: fix: `fill_match_arms` hangs on a tuple of large enums r=matklad a=iDawer
+ Lazy computation of missing arms.
+ Convenience function to test lazy computation: `ide_assists::tests::check_assist_unresolved`.
Fixes#8835
Co-authored-by: Dawer <7803845+iDawer@users.noreply.github.com>