pgsql: Preserve clustered index after rewrites with ALTER TABLE

Preserve clustered index after rewrites with ALTER TABLE

A table rewritten by ALTER TABLE would lose tracking of an index usable
for CLUSTER.  This setting is tracked by pg_index.indisclustered and is
controlled by ALTER TABLE, so some extra work was needed to restore it
properly.  Note that ALTER TABLE only marks the index that can be used
for clustering, and does not do the actual operation.

Author: Amit Langote, Justin Pryzby
Reviewed-by: Ibrar Ahmed, Michael Paquier
Discussion: https://postgr.es/m/20200202161718.GI13621@telsasoft.com
Backpatch-through: 9.5

Branch
------
REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3e62dd3a937b7da84e8ae126c7bf8319c1cd5c08

Modified Files
--------------
src/backend/commands/tablecmds.c          | 46 +++++++++++++++++++++++++++++
src/backend/utils/cache/lsyscache.c       | 23 +++++++++++++++
src/include/utils/lsyscache.h             |  1 +
src/test/regress/expected/alter_table.out | 48 +++++++++++++++++++++++++++++++
src/test/regress/sql/alter_table.sql      | 25 ++++++++++++++++
5 files changed, 143 insertions(+)