pgsql: Calculate agglevelsup correctly when Aggref contains a CTE.

Calculate agglevelsup correctly when Aggref contains a CTE.

If an aggregate function call contains a sub-select that has
an RTE referencing a CTE outside the aggregate, we must treat
that reference like a Var referencing the CTE's query level
for purposes of determining the aggregate's level.  Otherwise
we might reach the nonsensical conclusion that the aggregate
should be evaluated at some query level higher than the CTE,
ending in a planner error or a broken plan tree that causes
executor failures.

Bug: #19055
Reported-by: BugForge <dllggyx@outlook.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19055-6970cfa8556a394d@postgresql.org
Backpatch-through: 13

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/31bf0963208f1a5121a81aa7c17b4e55bfdc5596

Modified Files
--------------
src/backend/parser/parse_agg.c     | 28 +++++++++++++++++++++++++++-
src/test/regress/expected/with.out | 34 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/with.sql      | 14 ++++++++++++++
3 files changed, 75 insertions(+), 1 deletion(-)