pgsql: Fix rd_firstRelfilenodeSubid for nailed relations, in parallel w

Fix rd_firstRelfilenodeSubid for nailed relations, in parallel workers.

Move applicable code out of RelationBuildDesc(), which nailed relations
bypass.  Non-assert builds experienced no known problems.  Back-patch to
v13, where commit c6b92041d38512a4176ed76ad06f713d2e6c01a8 introduced
rd_firstRelfilenodeSubid.

Kyotaro Horiguchi.  Reported by Justin Pryzby.

Discussion: https://postgr.es/m/20200907023737.GA7158@telsasoft.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6f15be5bedfc423b8a8f2f0282e2a0eb20a16663

Modified Files
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src/backend/utils/cache/relcache.c            | 23 +++++++++++++++--------
src/test/regress/expected/reindex_catalog.out | 10 ++++++++++
src/test/regress/sql/reindex_catalog.sql      | 11 +++++++++++
3 files changed, 36 insertions(+), 8 deletions(-)