pgsql: Ensure that the result of evaluating a function during

Log Message:
-----------
Ensure that the result of evaluating a function during constant-expression
simplification gets detoasted before it is incorporated into a Const node.
Otherwise, if an immutable function were to return a TOAST pointer (an
unlikely case, but it can be made to happen), we would end up with a plan
that depends on the continued existence of the out-of-line toast datum.

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.249 -> r1.250)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.249&r2=1.250)