Fix dependency handling at swap phase of REINDEX CONCURRENTLY

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 on 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 partition indexes.

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