In Item_ref::fix_fields() do invoke mark_as_dependent() for outside 
references in all cases (see email for more details)

sql/item.cc:
  In Item_ref::fix_fields() do invoke mark_as_dependent() for outside references in all cases.
sql/item.h:
  Change Field_enumerator to enumerate Item_field-s not Field-s.
sql/item_subselect.cc:
  Change Field_enumerator to enumerate Item_field-s not Field-s.
sql/opt_table_elimination.cc:
  Change Field_enumerator to enumerate Item_field-s not Field-s.