pgsql: Fix mishandling of sortgroupref labels while splitting SRF targe

Fix mishandling of sortgroupref labels while splitting SRF targetlists.

split_pathtarget_at_srfs() neglected to worry about sortgroupref labels
in the intermediate PathTargets it constructs. I think we'd supposed
that their labeling didn't matter, but it does at least for the case that
GroupAggregate/GatherMerge nodes appear immediately under the ProjectSet
step(s). This results in "ERROR: ORDER/GROUP BY expression not found in
targetlist" during create_plan(), as reported by Rajkumar Raghuwanshi.

To fix, make this logic track the sortgroupref labeling of expressions,
not just their contents. This also restores the pre-v10 behavior that
separate GROUP BY expressions will be kept distinct even if they are
textually equal().

Discussion: /messages/by-id/CAKcux6=1_Ye9kx8YLBPmJs_xE72PPc6vNi5q2AOHowMaCWjJ2w@mail.gmail.com

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a4c95b0b80c70677c09c0d5c82a6fba875160288

Modified Files
--------------
src/backend/optimizer/util/tlist.c | 146 +++++++++++++++++++++++---
src/test/regress/expected/select_parallel.out | 62 +++++++++++
src/test/regress/sql/select_parallel.sql | 7 ++
3 files changed, 198 insertions(+), 17 deletions(-)