Make a distinction between (1) WHERE clause constraints, (2) ON/USING

Make a distinction between (1) WHERE clause constraints, (2) ON/USING
constraints on outer joins, and (3) ON/USING clause constraints on inner
joins.  Formerly, there was no distinctionb between 1 and 3, but RIGHT JOIN
needs to know the difference.  Make RIGHT JOIN aware of this difference and
add test cases.

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