Allow ALTER INDEX .. ATTACH PARTITION to validate a parent index

Allow ALTER INDEX .. ATTACH PARTITION to validate a parent index

This commit tweaks ALTER INDEX .. ATTACH PARTITION to attempt a
validation of a parent index in the case where an index is already
attached but the parent is not yet valid.  This occurs in cases where a
parent index was created invalid such as with CREATE INDEX ONLY, but was
left invalid after an invalid child index was attached (partitioned
indexes set indisvalid to false if at least one partition is
!indisvalid, indisvalid is true in a partitioned table iff all
partitions are indisvalid).  This could leave a partition tree in a
situation where a user could not bring the parent index back to valid
after fixing the child index, as there is no built-in mechanism to do
so.  This commit relies on the fact that repeated ATTACH PARTITION
commands on the same index silently succeed.

An invalid parent index is more than just a passive issue.  It causes
for example ON CONFLICT on a partitioned table if the invalid parent
index is used to enforce a unique constraint.

Some test cases are added to track some of problematic patterns, using a
set of partition trees with combinations of invalid indexes and ATTACH
PARTITION.

Reported-by: Mohamed Ali <moali.pg@gmail.com>
Author: Sami Imseih <sanmimseih@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Haibo Yan <tristan.yim@gmail.com>
Discussion: http://postgr.es/m/CAGnOmWqi1D9ycBgUeOGf6mOCd2Dcf=6sKhbf4sHLs5xAcKVCMQ@mail.gmail.com
Backpatch-through: 14