move callee get/set code from irnode to cgana
fix corner cases when determining called entities/irgs

Most firm analyses behaved incorrectlye when: A called SymConst did not
have a method entity, assumed the code reachable by get_entity_irg is
the code called. These assumptions are wrong when doing crazy casts in C
or when dealing with weak symbols. This is fixed by introducing some
convenience functions: get_Call_callee(), get_entity_linktime_irg()
further cleanup in get/set_entity_irg/additional_properties

- Only allow get_entity_irg()/set_entity_irg() for method entities now.
- Allow setting/getting of additional properties for method and alias
  entities.
- Add some missing is_method_entity() checks in cgana.