pgsql: Fix miscomputation of direct_lateral_relids for join relations.

Fix miscomputation of direct_lateral_relids for join relations.

If a PlaceHolderVar is to be evaluated at a join relation, but
its value is only needed there and not at higher levels, we neglected
to update the joinrel's direct_lateral_relids to include the PHV's
source rel. This causes problems because join_is_legal() then won't
allow joining the joinrel to the PHV's source rel at all, leading
to "failed to build any N-way joins" planner failures.

Per report from Andreas Seltenreich. Back-patch to 9.5
where the problem originated.

Discussion: https://postgr.es/m/87blfgqa4t.fsf@aurora.ydns.eu

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4f728a1f5f06dab85f6b94fd062752a25239c6b6

Modified Files
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src/backend/optimizer/util/placeholder.c | 38 +++++++++++++------
src/test/regress/expected/join.out | 64 ++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql | 27 ++++++++++++++
3 files changed, 118 insertions(+), 11 deletions(-)