internal/core/adt: fix cycle for structural cycles

One of the issues was that we should not use a
recursive Finalize for structs that are used
as a for source, as this would lead to triggering
a structural cycle. This means we currently
ignore recursive errors.

Another issue was (disovered by Claude) that
comprehensions need to be considered "optional"
to avoid triggering structural cycles in some
cases.

Issue #3940
Fixes #3990

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Iaa22f8725cffc1466709d87a504f50730f0c9b3e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1219948
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1220065
Reviewed-by: Roger Peppe <[email protected]>

Author: mpvl