pgsql: Disallow SRFs when considering sorts below Gather Merge

Disallow SRFs when considering sorts below Gather Merge

While we do allow SRFs in ORDER BY, scan/join processing should not
consider such cases - such sorts should only happen via final Sort atop
a ProjectSet. So make sure we don't try adding such sorts below Gather
Merge, just like we do for expressions that are volatile and/or not
parallel safe.

Backpatch to PostgreSQL 13, where this code was introduced as part of
the Incremental Sort patch.

Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13
Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com
Discussion: https://postgr.es/m/295524.1606246314%40sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d0167631e8b7388b78203c6798621f98beed93d5

Modified Files
--------------
src/backend/optimizer/path/equivclass.c        |  7 +++++++
src/backend/optimizer/util/tlist.c             |  5 -----
src/include/optimizer/optimizer.h              |  5 +++++
src/test/regress/expected/incremental_sort.out | 12 ++++++++++++
src/test/regress/sql/incremental_sort.sql      |  2 ++
5 files changed, 26 insertions(+), 5 deletions(-)