This commit adds partitioned indexes from upstream Postgres. This commit is
mostly cherry-picked from Postgres 11 and bug fixes from Postgres 12.

Differences from upstream:
 - Postgres has two additional relkind's - RELKIND_PARTITIONED_TABLE and
   RELKIND_PARTITIONED_INDEX, which have no on disk storage. Greenplum does not
   have these additional relkinds. Thus, partitioned indexes have physical
   storage.
 - CREATE INDEX ON ONLY <table> DDL has not yet been implemented
 - ALTER INDEX ATTACH PARTITION DDL has not yet been implemented

Constraint changes:
 - Constraints and their backing index have the same names. Thus, partitions of
   a table no longer share the same constraint name, and are instead related to
   their parent via INTERNAL_AUTO dependencies.

Index changes:
 - Child partition indexes can no longer be directly dropped, and must be
   dropped from their root. This includes mid-level and leaf indexes.
 - Adding indexes to mid-level partitions cascade to their children.

These changes are mostly cherry-picked from:
commit 8b08f7d4
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date:   Fri Jan 19 11:49:22 2018 -0300

    Local partitioned indexes

    When CREATE INDEX is run on a partitioned table, create catalog entries
    for an index on the partitioned table (which is just a placeholder since
    the table proper has no data of its own), and recurse to create actual
    indexes on the existing partitions; create them in future partitions
    also.

    As a convenience gadget, if the new index definition matches some
    existing index in partitions, these are picked up and used instead of
    creating new ones.  Whichever way these indexes come about, they become
    attached to the index on the parent table and are dropped alongside it,
    and cannot be dropped on isolation unless they are detached first.

    To support pg_dump'ing these indexes, add commands
        CREATE INDEX ON ONLY <table>
    (which creates the index on the parent partitioned table, without
    recursing) and
        ALTER INDEX ATTACH PARTITION
    (which is used after the indexes have been created individually on each
    partition, to attach them to the parent index).  These reconstruct prior
    database state exactly.

    Reviewed-by: (in alphabetical order) Peter Eisentraut, Robert Haas, Amit
            Langote, Jesper Pedersen, Simon Riggs, David Rowley
    Discussion: https://postgr.es/m/20171113170646.gzweigyrgg6pwsg4@alvherre.pgsql

Changes were also cherry-picked from the following Postgres commits:
  eb7ed3f3 - Allow UNIQUE indexes on partitioned tables
  ae366aa5 - Detach constraints when partitions are detached
  19184fcc - Simplify coding to detach constraints when detaching partition
  c7d43c4d - Correct attach/detach logic for FKs in partitions
  17f206fb - Set pg_class.relhassubclass for partitioned indexes
Co-authored-by: NKalen Krempely <kkrempely@pivotal.io>