internal/core/adt: fix closedness bug with patterns

When a pattern constraint is matched to a field, this field
is added to the closeContext of the corresponding pattern.
However, this does not mean that the field is actually present.
This distinction is relevant when later down the line, two
pattern constraints are intersected, invalidating a field.
In order to track this, we mark an arc as "matched", as
opposed to explicit.

Issue #3601

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: If283780babdcdd03723ae84d3b2af2f6f1fbed48
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205363
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>