Set pg_class.relhassubclass for partitioned indexes

Set pg_class.relhassubclass for partitioned indexes

Like for relations, switching this parameter is optimistic by turning it
on each time a partitioned index gains a partition.  So seeing this
parameter set to true means that the partitioned index has or has had
partitions.  The flag cannot be reset yet for partitioned indexes, which
is something not obvious anyway as partitioned relations exist to have
partitions.

This allows to track more conveniently partition trees for indexes,
which will come in use with an upcoming patch helping in listing
partition trees with an SQL-callable function.

Author: Amit Langote
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/80306490-b5fc-ea34-4427-f29c52156052@lab.ntt.co.jp