ATTACH PARTITION: Don't match a PK with a UNIQUE constraint

When matching constraints in AttachPartitionEnsureIndexes() we weren't
testing the constraint type, which could make a UNIQUE key lacking a
not-null constraint incorrectly satisfy a primary key requirement.  Fix
this by testing that the constraint types match.  (Other possible
mismatches are verified by comparing index properties.)

Discussion: https://postgr.es/m/202402051447.wimb4xmtiiyb@alvherre.pgsql

src/backend/utils/cache/lsyscache.c

@@ -1133,6 +1133,28 @@ get_constraint_index(Oid conoid)
 		return InvalidOid;
 }
 
+/*
+ * get_constraint_type
+ *		Return the pg_constraint.contype value for the given constraint.
+ *
+ * No frills.
+ */
+char
+get_constraint_type(Oid conoid)
+{
+	HeapTuple	tp;
+	char		contype;
+
+	tp = SearchSysCache1(CONSTROID, ObjectIdGetDatum(conoid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for constraint %u", conoid);
+
+	contype = ((Form_pg_constraint) GETSTRUCT(tp))->contype;
+	ReleaseSysCache(tp);
+
+	return contype;
+}
+
 /*				---------- LANGUAGE CACHE ----------					 */
 
 char *