pgsql: Disallow extended statistics on system columns

Disallow extended statistics on system columns

Since introduction of extended statistics, we've disallowed references
to system columns. So for example

CREATE STATISTICS s ON ctid FROM t;

would fail. But with extended statistics on expressions, it was possible
to work around this limitation quite easily

CREATE STATISTICS s ON (ctid::text) FROM t;

This is an oversight in a4d75c86bf, fixed by adding a simple check.
Backpatch to PostgreSQL 14, where support for extended statistics on
expressions was introduced.

Backpatch-through: 14
Discussion: https://postgr.es/m/20210816013255.GS10479%40telsasoft.com

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/66061077155d68463ec00604ba7d6f0ae69716e8

Modified Files
--------------
src/backend/commands/statscmds.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)