pgsql: Fix more issues with dependency handling at swap phase of REINDE

Fix more issues with dependency handling at swap phase of REINDEX CONCURRENTLY

When canceling a REINDEX CONCURRENTLY operation after swapping is done,
a drop of the parent table would leave behind old indexes. This is a
consequence of 68ac9cf, which fixed the case of pg_depend bloat when
repeating REINDEX CONCURRENTLY on the same relation.

In order to take care of the problem without breaking the previous fix,
this uses a different strategy, possible even with the exiting set of
routines to handle dependency changes. The dependencies of/on the
new index are additionally switched to the old one, allowing an old
invalid index remaining around because of a cancellation or a failure to
use the dependency links of the concurrently-created index. This
ensures that dropping any objects the old invalid index depends on also
drops the old index automatically.

Reported-by: Julien Rouhaud
Author: Michael Paquier
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/20200227080735.l32fqcauy73lon7o@nol
Backpatch-through: 12

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/26876127be2cbbb3ecb18bf75c0670bfb1a3d289

Modified Files
--------------
src/backend/catalog/index.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)