pgsql: Preserve pg_index.indisclustered across REINDEX CONCURRENTLY

Preserve pg_index.indisclustered across REINDEX CONCURRENTLY

If the flag value is lost, a CLUSTER query following REINDEX
CONCURRENTLY could fail. Non-concurrent REINDEX is already handling
this case consistently.

Author: Justin Pryzby
Discussion: /messages/by-id/20200229024202.GH29456@telsasoft.com
Backpatch-through: 12

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d79fb88ac738e0854e84a6c9445babfa5b2504b0

Modified Files
--------------
src/backend/catalog/index.c | 8 +++++++-
src/test/regress/expected/create_index.out | 13 +++++++++++++
src/test/regress/sql/create_index.sql | 9 +++++++++
3 files changed, 29 insertions(+), 1 deletion(-)