Fix dependency handling at swap phase of REINDEX CONCURRENTLY

When swapping the dependencies of the old and new indexes, the code has
been correctly switching all links in pg_depend from the old to the new
index for both referencing and referenced entries.  However it forgot
the fact that the new index may itself have existing entries in
pg_depend, like references to the parent table attributes.  This
resulted in duplicated entries in pg_depend after running REINDEX
CONCURRENTLY.

Fix this problem by removing any existing entries in pg_depend for the
new index before switching the dependencies of the old index to the new
one.  More regression tests are added to check the consistency of
entries in pg_depend for indexes, including partitions.

Author: Michael Paquier
Discussion: https://postgr.es/m/20191025064318.GF8671@paquier.xyz
Backpatch-through: 12