define a new structure type index - index of new structure type If another structure has the specified index, then index of that structure and all other structures will be increment freeing the specified index. If index is == -1, then the biggest index number will be used. See get_first_struc_idx() for the explanation of structure indices and IDs.
returns: -1 if can't define structure type because of bad structure name: the name is ill-formed or is already used in the program. otherwise returns ID of the new structure type
long add_struc(long index, string name, long is_union);