INSERT...ON DUPLICATE KEY IGNORE RETURNING REJECTS

It was previously possible to have RETURNING return "affected rows" in respect
of all INSERT STATEMENTS; this could easily be used to indicate which rows were
successfully inserted when combined with ON DUPLICATE KEY IGNORE.

However, in practice I anticipate that it'll be more useful to return rows that
were rejected, as when inserting data in bulk of dubious quality. So create a
new parsenode representation of RETURNING, ReturningClause, and have that carry
around whether or not the new REJECTS reserved keyword was specified.  If so,
return rows that were rejected, not affected by the speculative insertion -
exactly the inverse of the default.

This is only meaningful in the context of INSERT...ON DUPLICATE KEY IGNORE.