pgsql: Allow USING and INTO clauses of plpgsql's EXECUTE to appear in

Log Message:
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Allow USING and INTO clauses of plpgsql's EXECUTE to appear in either order.

Aside from being more forgiving, this prevents a rather surprising misbehavior
when the "wrong" order was used: the old code didn't throw a syntax error,
but absorbed the INTO clause into the last USING expression, which then did
strange things downstream.

Intentionally not changing the documentation; we'll continue to advertise
only the "standard" clause order.

Backpatch to 8.4, where the USING clause was added to EXECUTE.

Modified Files:
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pgsql/src/pl/plpgsql/src:
gram.y (r1.143 -> r1.144)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.143&r2=1.144)