Allowing GROUP_CONCAT(... ORDER BY ..) in queries with ROLLUP.

The story of the related code:

1. The original patch from Wax
commit: 0b505fb4
date: Tue Mar 18 03:07:40 2003

opt_gorder_clause reused the regular order_clause,
which already had some protection against ROLLUP queries:

order_clause:
        ORDER_SYM BY
        {
          LEX *lex=Lex;
          if (lex->current_select->linkage != GLOBAL_OPTIONS_TYPE &&
              lex->current_select->select_lex()->olap !=
              UNSPECIFIED_OLAP_TYPE)
          {
            net_printf(lex->thd, ER_WRONG_USAGE,
                       "CUBE/ROLLUP",
                       "ORDER BY");
            YYABORT;
          }
        } order_list;

The assumption that ORDER BY in group_concat() had to have
the same ROLLUP restriction (with order_clause) was wrong.

Moreover, GROUP_CONCAT() in select_item_list was not affected
by this restriction, because WITH ROLLUP goes after
select_item_list and therefore sel->olap is always equal
to UNSPECIFIED_OLAP_TYPE during select_item_list.

GROUP BY was not affected for two reasons:
- it goes before WITH ROLLUP and sel->olap is still
  UNSPECIFIED_OLAP_TYPE
- Aggregate functions like AVG(), GROUP_CONCAT() in GROUP BY
  are not allowed

So only GROUP_CONCAT() in HAVING and ORDER BY clauses
were erroneously affected by this restriction.

2. Bug#27848 rollup in union part causes error with order of union
commit: 3f6073ae
Author: unknown <igor@olga.mysql.com>  2007-12-15 01:42:46

The condition in the ROLLUP protection code became more complex.
Note, opt_gconcat_order still reused the regular order_clause.

3. Bug#16347426 ASSERTION FAILED: (SELECT_INSERT &&
                 !TABLES->NEXT_NAME_RESOLUTION_TABLE) || !TAB
commit: 2d836633
author: Chaithra Gopalareddy <chaithra.gopalareddy@oracle.com>
date: 2013-04-14 06:00:49

opt_gorder_clause was refactored not to use order_clause and
to collect information directly to select->gorder_list.
The ROLLUP protection code was duplicated from order_clause
to the new version of opt_gorder_clause.