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
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e7f9f44e3bd95404f4221fab29cd295805ff410d

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(-)