Stop skipping const items when selecting but skip them when storing
their results to spider row to avoid storing in mismatching temporary
table fields.
Skip auxiliary fields in SELECTing, and do not store
the (non-existing) results to the corresponding temporary table
accordingly.
When there are BOTH auxiliary fields AND const items in the auxiliary
field items, do not use the spider GBH. This is a rare occasion if it
happens at all and not worth the added complexity to cover it.
Use the original item (item_ptr) in constructing GROUP BY and ORDER
BY, which also means using item->name instead of field->field_name as
aliases in constructing SELECT items. This fixes spurious regressions
caused by the above changes in some tests using ORDER BY, such as
mdev_24517.test. As a by-product, this also fixes MDEV-29546.
Therefore we update mdev_29008.test to include the MDEV-29546 case.
