This bug manifested itself when executing queries with multiple reference
to a CTE specified by a query expression with union and having its
column names explicitly declared. In this case the server returned a bogus
error message about unknown column name. It happened because while for the
first reference to the CTE the names of the columns returned by the CTE
specification were properly changed to match the CTE definition for the
other references it was not done. This was a consequence  of not quite
complete code of the function With_element::clone_parsed_spec() that forgot
to set the reference to the CTE definition for unit structures representing
non-first CTE references.

Approved by dmitry.shulga@mariadb.com