internal/core/adt: match decrement for nested comprehension arcs

The current logic for closedness counter matching did not account
for nested arcs in comprehensions, causing errors to be reported
when adt.DebugDeps was set to false.

This does not fix any tests, but it will reduce the number of errors
reported setting DebugDeps to true, which is currently set to false.

Issue #2884

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ie3efec2d30bdab0ea6802cf9d1f978e5a51abdc3
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193641
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

internal/core/adt/comprehension.go

@@ -151,6 +151,7 @@ func (n *nodeContext) insertComprehension(
 	}
 
 	if ec.done && len(ec.envs) == 0 {
+		n.decComprehension(c)
 		return
 	}
 
@@ -408,14 +409,21 @@ func (n *nodeContext) processComprehension(d *envYield, state vertexStatus) *Bot
 	// NOTE: we cannot move this to defer in processComprehensionInner, as we
 	// use panics to implement "yielding" (and possibly coroutines in the
 	// future).
-	cc := d.leaf.cc
-	if cc != nil {
-		cc.decDependent(n.ctx, COMP, d.leaf.arcCC)
-	}
-	d.leaf.cc = nil
+	n.decComprehension(d.leaf)
+
 	return err
 }
 
+func (n *nodeContext) decComprehension(p *Comprehension) {
+	for ; p != nil; p = p.parent {
+		cc := p.cc
+		if cc != nil {
+			cc.decDependent(n.ctx, COMP, p.arcCC)
+		}
+		p.cc = nil
+	}
+}
+
 func (n *nodeContext) processComprehensionInner(d *envYield, state vertexStatus) *Bottom {
 	ctx := n.ctx