add array_comprehension case to to_binding_expr

As identified in #6810, to_binding_expr could also cover array_comprehension
expressions.

Author: kroening

src/util/std_expr.h

@@ -3090,7 +3090,8 @@ inline void validate_expr(const binding_exprt &binding_expr)
 inline const binding_exprt &to_binding_expr(const exprt &expr)
 {
   PRECONDITION(
-    expr.id() == ID_forall || expr.id() == ID_exists || expr.id() == ID_lambda);
+    expr.id() == ID_forall || expr.id() == ID_exists ||
+    expr.id() == ID_lambda || expr.id() == ID_array_comprehension);
   const binding_exprt &ret = static_cast<const binding_exprt &>(expr);
   validate_expr(ret);
   return ret;
@@ -3105,7 +3106,8 @@ inline const binding_exprt &to_binding_expr(const exprt &expr)
 inline binding_exprt &to_binding_expr(exprt &expr)
 {
   PRECONDITION(
-    expr.id() == ID_forall || expr.id() == ID_exists || expr.id() == ID_lambda);
+    expr.id() == ID_forall || expr.id() == ID_exists ||
+    expr.id() == ID_lambda || expr.id() == ID_array_comprehension);
   binding_exprt &ret = static_cast<binding_exprt &>(expr);
   validate_expr(ret);
   return ret;