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: https://postgr.es/m/
20200229024202.GH29456@telsasoft.com
Backpatch-through: 12