Fix a problem with the query optimizer for LIMIT/OFFSET queries when

underlying query is a UNION ALL and both arms of the UNION ALL are
subqueries with an ORDER BY clause.  This bug was reported at
[forum:/forumpost/6b5e9188f0657616|forum post 6b5e9188f0657616].  The
problem was introduced in 2015 (SQLite version 3.9.0) by check-in
[4b631364354068af].  See also ticket [b65cb2c8d91f6685].

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