only const tables

The problem was caused by two shortcuts in the optimizer that
are inapplicable in the ROLLUP case.

Normally in a case when only const tables are involved in a
query, DISTINCT clause can be safely optimized away since there
may be only one row produced by the join. Similarly, we don't
need to create a temporary table to resolve DISTINCT/GROUP
BY/ORDER BY. Both of these are inapplicable when the WITH
ROLLUP modifier is present.

Fixed by disabling the said optimizations for the WITH ROLLUP
case.

mysql-test/r/olap.result:
  Added a test case for bug #48475.
mysql-test/t/olap.test:
  Added a test case for bug #48475.
sql/sql_select.cc:
  Disabled const-only table optimizations for the WITH ROLLUP
  case.