pgsql: Avoid unhelpful internal error for incorrect recursive-WITH quer

Avoid unhelpful internal error for incorrect recursive-WITH queries.

checkWellFormedRecursion would issue "missing recursive reference"
if a WITH RECURSIVE query contained a single self-reference but
that self-reference was inside a top-level WITH, ORDER BY, LIMIT,
etc, rather than inside the second arm of the UNION as expected.
We already intended to throw more-on-point errors for such cases,
but those error checks must be done before examining the UNION arm
in order to have the desired results. So this patch need only
move some code (and improve the comments).

Per bug #18536 from Alexander Lakhin. Back-patch to all supported
branches.

Discussion: https://postgr.es/m/18536-0a342ec07901203e@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f96c2c72788cab3005c8bc6b4934b4928b34a529

Modified Files
--------------
src/backend/parser/parse_cte.c | 53 ++++++++++++++++++++++++--------------
src/test/regress/expected/with.out | 40 ++++++++++++++++++++++++++++
src/test/regress/sql/with.sql | 29 +++++++++++++++++++++
3 files changed, 102 insertions(+), 20 deletions(-)