pgsql: Disallow whole-row variables in GENERATED expressions.

Disallow whole-row variables in GENERATED expressions.

This was previously allowed, but I think that was just an oversight.
It's a clear violation of the rule that a generated column cannot
depend on itself or other generated columns.  Moreover, because the
code was relying on the assumption that no such cross-references
exist, it was pretty easy to crash ALTER TABLE and perhaps other
places.  Even if you managed not to crash, you got quite unstable,
implementation-dependent results.

Per report from Vitaly Ustinov.
Back-patch to v12 where GENERATED came in.

Discussion: https://postgr.es/m/CAM_DEiWR2DPT6U4xb-Ehigozzd3n3G37ZB1+867zbsEVtYoJww@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4b10074453d182b5fc11a5667bab2ef8532ff3a6

Modified Files
--------------
src/backend/catalog/heap.c              | 15 +++++++++++++--
src/test/regress/expected/generated.out |  7 +++++++
src/test/regress/sql/generated.sql      |  3 +++
3 files changed, 23 insertions(+), 2 deletions(-)