Fix lost persistence setting during REINDEX INDEX

ReindexIndex() trusts a parser-built RangeVar with the persistence to
use for the new copy of the index; but the parser naturally does not
know what's the persistence of the original index.  To find out the
correct persistence, grab it from relcache.

This bug was introduced by commit 85b506bb, and therefore no
backpatch is necessary.

Bug reported by Thom Brown, analysis and patch by Michael Paquier; test
case provided by Fabrízio de Royes Mello.