Before this fix, a IN predicate of the form: "IN (( subselect ))", with two
parenthesis, would be evaluated as a single row subselect: if the subselect
returns more that 1 row, the statement would fail.

The SQL:2003 standard defines a special exception in the specification,
and mandates that this particular form of IN predicate shall be equivalent
to "IN ( subselect )", which involves a table subquery and works with more
than 1 row.

This fix implements "IN (( subselect ))", "IN ((( subselect )))" etc
as per the SQL:2003 requirement.

All the details related to the implementation of this change have been
commented in the code, and the relevant sections of the SQL:2003 spec
are given for reference, so they are not repeated here.

Having access to the spec is a requirement to review in depth this patch.


mysql-test/r/subselect.result:
  Implement IN predicate special exceptions with subselects.
mysql-test/t/subselect.test:
  Implement IN predicate special exceptions with subselects.
sql/item_subselect.cc:
  Implement IN predicate special exceptions with subselects.
sql/item_subselect.h:
  Implement IN predicate special exceptions with subselects.
sql/sql_yacc.yy:
  Implement IN predicate special exceptions with subselects, cleanup.