Drop region parameter to Unbounded, as it can be done without.

src/effects.h

Index: src/effects.h
diff --git a/src/effects.h b/src/effects.h
index e18baeafd6f47ee2b4bcb2e1b9895602661ee653..8d539f64bd8f64d11032353c6d8365a320c70446 100644
--- a/src/effects.h
+++ b/src/effects.h
@@ -35,11 +35,11 @@ struct Effect {
 
   // The unknown effect.
   static Effect Unknown(Zone* zone) {
-    return Effect(Bounds::Unbounded(zone), POSSIBLE);
+    return Effect(Bounds::Unbounded(), POSSIBLE);
   }
 
   static Effect Forget(Zone* zone) {
-    return Effect(Bounds::Unbounded(zone), DEFINITE);
+    return Effect(Bounds::Unbounded(), DEFINITE);
   }
 
   // Sequential composition, as in 'e1; e2'.
@@ -87,7 +87,7 @@ class EffectsMixin: public Base {
   Bounds LookupBounds(Var var) {
     Effect effect = Lookup(var);
     return effect.modality == Effect::DEFINITE
-        ? effect.bounds : Bounds::Unbounded(Base::zone());
+        ? effect.bounds : Bounds::Unbounded();
   }
 
   // Sequential composition.