When the SQL_BIG_RESULT flag is specified SELECT should store items from the
select list in the filesort data and use them when sending to a client.
The get_addon_fields function is responsible for creating necessary structures
for that. But this function was allowed to do so only for SELECT and
INSERT .. SELECT queries. This makes the SQL_BIG_RESULT useless for
the CREATE .. SELECT queries.

Now the get_addon_fields allows storing select list items in the filesort
data for the CREATE .. SELECT queries.