Fix RLS checks for FOR PORTION OF leftover rows

UPDATE/DELETE FOR PORTION OF may insert leftover rows to preserve the
parts of the old row that are outside the target range.  Those inserts
go through ExecInsert(), which checks RLS policies using
WCO_RLS_INSERT_CHECK.

However, the rewriter only added RLS WITH CHECK options for the
original statement command.  For UPDATE, that meant only
WCO_RLS_UPDATE_CHECK options were available, so ExecInsert() skipped
them.  For DELETE, no RLS WITH CHECK options were added at all.  As a
result, leftover rows could be inserted even when they violated INSERT
RLS policies.

Fix this by adding INSERT RLS WITH CHECK options for UPDATE/DELETE FOR
PORTION OF target relations.  Also add regression coverage for both
UPDATE and DELETE, including cases where allowed leftovers still
succeed and disallowed leftovers are rejected.

Author: Chao Li <[email protected]>
Co-authored-by: Paul A Jungwirth <[email protected]>
Reviewed-by: Paul A Jungwirth <[email protected]>
Reviewed-by: Ayush Tiwari <[email protected]>
Reviewed-by: Dean Rasheed <[email protected]>
Discussion: 
https://www.postgresql.org/message-id/flat/6C34A987-AC50-4477-BD71-2D4AFEE1A589%40gmail.com
Discussion: 
https://www.postgresql.org/message-id/flat/CAJTYsWWdeBkoH5g8D-k9LDw9ciqsMxb21EJSiFXAzP4J%3DXyxOQ%40mail.gmail.com

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c58c83ce50cf65ee45bd60910f710a2bd251cc36

Modified Files
--------------
doc/src/sgml/ref/create_policy.sgml          | 32 ++++++++++++
doc/src/sgml/ref/delete.sgml                 |  3 +-
doc/src/sgml/ref/update.sgml                 |  3 +-
src/backend/rewrite/rowsecurity.c            | 25 ++++++++++
src/test/regress/expected/for_portion_of.out | 75 ++++++++++++++++++++++++++++
src/test/regress/expected/rowsecurity.out    | 39 ++++++++++++++-
src/test/regress/sql/for_portion_of.sql      | 58 +++++++++++++++++++++
src/test/regress/sql/rowsecurity.sql         | 33 +++++++++++-
8 files changed, 264 insertions(+), 4 deletions(-)

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