context was used as an argument of GROUP_CONCAT.
Ensured correct setting of the depended_from field in references
generated for set functions aggregated in outer selects.
A wrong value of this field resulted in wrong maps returned by 
used_tables() for these references.
Made sure that a temporary table field is added for any set function
aggregated in outer context when creation of a temporary table is 
needed to execute the inner subquery.