EXPLAIN UNION using same routing which used for execution which allow return correct bug messages (Bug #3639)
EXPLAIN of hidden SELECT of UNION


mysql-test/r/derived.result:
  explain of hidden select
mysql-test/r/subselect.result:
  explain of hidden select
mysql-test/r/union.result:
  explain of hidden select
  correct error messages on explain
mysql-test/t/subselect.test:
  show eliminated costants in WHERE clause
mysql-test/t/union.test:
  correct error messages on EXPLAIN with union
sql/item.cc:
  fixed name constructing for global ORDER BY items
sql/sql_class.h:
  select ID can be negative (for hidden SELECTs)
  removed unused field
sql/sql_lex.cc:
  new flag of UNION EXPLAIN
sql/sql_lex.h:
  new flag of UNION EXPLAIN
  select ID can be negative (for hidden SELECTs)
sql/sql_select.cc:
  EXPLAIN UNION using same routing which used for execution
  explain for hidden SELECT of UNION
sql/sql_union.cc:
  EXPLAIN UNION using same routing which used for execution