Presence of a subquery in the ON expression of a join 
should not block merging the view that contains this join.
Before this patch the such views were converted into 
into temporary table views.


mysql-test/r/view.result:
  Added a test case for bug #21646.
mysql-test/t/view.test:
  Added a test case for bug #21646.
sql/mysql_priv.h:
  Fixed bug #21646.
  Added a new parsing state 'IN_ON', true when
  the parser is in an ON expression of a join.
sql/sql_lex.cc:
  Fixed bug #21646.
  Presence of a subquery in the ON expression of a join 
  should not block merging the view that contains this join.
sql/sql_yacc.yy:
  Fixed bug #21646.
  Added a new parsing state 'IN_ON', true when
  the parser is in an ON expression of a join.