Now that we're tracking formals on Lambda this ends up being quite easy;
we just pull them off of the Lambda for the argument closure and use
them to construct the result attribute set.
Change-Id: I811cb61ec34c6bef123a4043000b18c0e4ea0125
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7003
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
