pgsql: Create FKs properly when attaching table as partition

Create FKs properly when attaching table as partition

Commit f56f8f8da6af added some code in CloneFkReferencing that's way too
lax about a Constraint node it manufactures, not initializing enough
struct members -- initially_valid in particular was forgotten. This
causes some FKs in partitions added by ALTER TABLE ATTACH PARTITION to
be marked as not validated. Set initially_valid true, which fixes the
bug.

While at it, make the struct initialization more complete. Very similar
code was added in two other places by the same commit; make them all
follow the same pattern for consistency, though no bugs are apparent
there.

This bug has never been reported: I only happened to notice while
working on commit 614a406b4ff1. The test case that was added there with
the improper result is repaired.

Backpatch to 12.

Discussion: https://postgr.es/m/20221005105523.bhuhkdx4olajboof@alvherre.pgsql

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ab70b3a52fde6f0519fc74f1aae1d7ad88cb16dd

Modified Files
--------------
src/backend/commands/tablecmds.c | 43 +++++++++++++++++++++++++------
src/test/regress/expected/foreign_key.out | 4 +--
2 files changed, 37 insertions(+), 10 deletions(-)