[ARROW-16631] [C++] Support order by derived column - ASF JIRA

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You can't do the equivalent of

SELECT x, y FROM table ORDER BY x * y ASC;

because sorting requires a named column AND because sorting is only done in a SinkNode. You can project to

{x, y, x*y}

then sort on x*y, but you can't then project back to

{x, y}

on the sorted data because that's a new ExecPlan and order is not preserved. In R we have to handle this outside of an ExecPlan.