Make object_factory_parameters::set virtual

This enables derived classes to implement their own language specific
options

Author: hannes-steffenhagen-diffblue

src/util/object_factory_parameters.h

@@ -77,7 +77,7 @@ struct object_factory_parameterst
   irep_idt function_id;
 
   /// Assigns the parameters from given options
-  void set(const optionst &);
+  virtual void set(const optionst &);
 };
 
 void parse_object_factory_options(const cmdlinet &, optionst &);