Ensure relcache consistency around generated columns

In certain transient states, it's possible that a table has attributes
with attgenerated set but no default expressions in pg_attrdef yet.
In that case, the old code path would not set
relation->rd_att->constr->has_generated_stored, unless
relation->rd_att->constr was also populated for some other reason.
There was probably no practical impact, but it's better to keep this
consistent.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/20200115181105.ad6ab6dlgyww3lb6%40alap3.anarazel.de

src/backend/utils/cache/relcache.c

@@ -669,8 +669,11 @@ RelationBuildTupleDesc(Relation relation)
 	/*
 	 * Set up constraint/default info
 	 */
-	if (constr->has_not_null || ndef > 0 ||
-		attrmiss || relation->rd_rel->relchecks)
+	if (constr->has_not_null ||
+		constr->has_generated_stored ||
+		ndef > 0 ||
+		attrmiss ||
+		relation->rd_rel->relchecks)
 	{
 		relation->rd_att->constr = constr;