pgsql: Avoid creating a RESULT RTE that's marked LATERAL.

Avoid creating a RESULT RTE that's marked LATERAL.

Commit 7266d0997 added code to pull up simple constant function
results, converting the RTE_FUNCTION RTE to a dummy RTE_RESULT
RTE since it no longer need be scanned. But I forgot to clear
the LATERAL flag if the RTE has it set. If the function reduced
to a constant, it surely contains no lateral references so this
simplification is logically OK. It's needed because various other
places will Assert that RESULT RTEs aren't LATERAL.

Per bug #17097 from Yaoguang Chen. Back-patch to v13 where the
faulty code came in.

Discussion: https://postgr.es/m/[email protected]

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9807b9aedc6e8b8a497db161b3d2d9f83648ea5b

Modified Files
--------------
src/backend/optimizer/prep/prepjointree.c | 5 ++++-
src/test/regress/expected/join.out | 8 ++++++++
src/test/regress/sql/join.sql | 3 +++
3 files changed, 15 insertions(+), 1 deletion(-)