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_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7df74e635e65e880c308d52e11d3d603aa1ee9c6

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(-)