pgsql: Disallow generated columns in COPY WHERE clause

Disallow generated columns in COPY WHERE clause

Stored generated columns are not yet computed when the filtering
happens, so we need to prohibit them to avoid incorrect behavior.

Co-authored-by: jian he <jian.universality@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CACJufxHb8YPQ095R_pYDr77W9XKNaXg5Rzy-WP525mkq+hRM3g@mail.gmail.com

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/26958f4d99b16b1c638e6d43d46623e9c79579d5

Modified Files
--------------
src/backend/commands/copy.c             | 37 +++++++++++++++++++++++++++++++++
src/test/regress/expected/generated.out |  6 ++++++
src/test/regress/sql/generated.sql      |  4 ++++
3 files changed, 47 insertions(+)