UNION over correlated and uncorrelated SELECTS.
In such subqueries each uncorrelated SELECT should be considered as
uncacheable. Otherwise join_free is called for it and in many cases
it causes some problems.


mysql-test/r/subselect.result:
  Added a test case for bug #25219.
mysql-test/t/subselect.test:
  Added a test case for bug #25219.
sql/mysql_priv.h:
  Fixed bug #25219: crash for a query that contains an EXIST subquery with
  UNION over correlated and uncorrelated SELECTS.
  In such subqueries each uncorrelated SELECT should be considered as
  uncacheable. Otherwise join_free is called for it and in many cases
  it causes some problems. 
  Added a new flag UNCACHEABLE_UNITED for such SELECTs.
sql/sql_lex.cc:
  Fixed bug #25219: crash for a query that contains an EXIST subquery with
  UNION over correlated and uncorrelated SELECTS.
  In such subqueries each uncorrelated SELECT should be considered as
  uncacheable. Otherwise join_free is called for it and in many cases
  it causes some problems.
  Added a new flag UNCACHEABLE_UNITED for such SELECTs.