The query was re-written *after* we had tagged it with NON_AGG_FIELD_USED.
Remove the flag before continuing.


mysql-test/r/explain.result:
  Update test case for Bug#48295.
mysql-test/r/subselect.result:
  New test case.
mysql-test/t/explain.test:
  Update test case for Bug#48295.
mysql-test/t/subselect.test:
  New test case.
sql/item.cc:
  Use accessor functions for non_agg_field_used/agg_func_used.
sql/item_subselect.cc:
  Remove non_agg_field_used when we rewrite query '1 < some (...)' => '1 < max(...)'
sql/item_sum.cc:
  Use accessor functions for non_agg_field_used/agg_func_used.
sql/mysql_priv.h:
  Remove unused #defines.
sql/sql_lex.cc:
  Initialize new member variables.
sql/sql_lex.h:
  Replace full_group_by_flag with two boolean flags,
  and itroduce accessors for manipulating them.
sql/sql_select.cc:
  Use accessor functions for non_agg_field_used/agg_func_used.