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