Add accessors for Item_sum and Item_func_group_concat classes.

class Item_sum :public Item_func_or_sum

...

Item **get_orig_args() { return orig_args; }

class Item_func_group_concat: Item_sum

...

bool is_distinct() { return distinct; }

uint count_field() { return arg_count_field; }

uint order_field() { return arg_count_order; }

String* str_separator() { return separator; }

ORDER** get_order() { return order; }