Update the mock domain with the new ai_domain_baset interface.

Author: martin

unit/analyses/ai/ai_simplify_lhs.cpp

@@ -33,6 +33,16 @@ class constant_simplification_mockt:public ai_domain_baset
   {}
   void make_entry() override
   {}
+  bool is_bottom() const override
+  {
+    UNREACHABLE;
+    return true;
+  }
+  bool is_top() const override
+  {
+    UNREACHABLE;
+    return true;
+  }
 
   bool ai_simplify(exprt &condition, const namespacet &ns) const override;
 };