internal/core/adt: fix edge case in structural cycles with structure sharing

In some cases, an ancestor node may be shared
in a child node. This bypasses the usual structure
sharing checks. This adds the check to catch this
edge case. We should probably find something more
principled down the line.

This problem was discovered as an issue when
debugging
Issue #3731

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ib5349a113088f82dfcb0c982a9ff0957c8af545f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209390
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl