pgsql: Flush table's relcache during ALTER TABLE ADD PRIMARY KEY USING

Flush table's relcache during ALTER TABLE ADD PRIMARY KEY USING INDEX.

Previously, unless we had to add a NOT NULL constraint to the column,
this command resulted in updating only the index's relcache entry.
That's problematic when replication behavior is being driven off the
existence of a primary key: other sessions (and ours too for that
matter) failed to recalculate their opinion of whether the table can
be replicated. Add a relcache invalidation to fix it.

This has been broken since pg_class.relhaspkey was removed in v11.
Before that, updating the table's relhaspkey value sufficed to cause
a cache flush. Hence, backpatch to v11.

Report and patch by Hou Zhijie

Discussion: https://postgr.es/m/OS0PR01MB5716EBE01F112C62F8F9B786947B9@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e2d53c8767da9f98c7c4c8d0b129171848472c93

Modified Files
--------------
src/backend/catalog/index.c | 11 +++++++++++
src/test/regress/expected/publication.out | 14 ++++++++++++++
src/test/regress/sql/publication.sql | 13 +++++++++++++
3 files changed, 38 insertions(+)