sql: allow UNION with hidden and non-hidden columns

This commit removes an assertion that required corresponding columns on
each side of a UNION to be both hidden or non-hidden.

Prior to this change, the following statements would yield the error:
"UNION types cannot be matched".

  CREATE TABLE ab (a INT, b INT);
  SELECT a, b, rowid FROM ab UNION VALUES (1, 2, 3);

With this commit, the above statements are executed without error.

Release note (bug fix): Fixed an incorrect error the ocurred when
executing UNION statements with hidden and non-hidden columns.

pkg/sql/logictest/testdata/logic_test/union

@@ -343,3 +343,8 @@ query T rowsort
 {2}
 NULL
 {3}
+
+# Allow UNION of hidden and non-hidden columns.
+statement ok
+CREATE TABLE ab (a INT, b INT);
+SELECT a, b, rowid FROM ab UNION VALUES (1, 2, 3);

pkg/sql/union.go

@@ -12,7 +12,6 @@ package sql
 
 import (
 	"context"
-	"fmt"
 
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgerror"
@@ -114,9 +113,6 @@ func (p *planner) newUnionNode(
 			return nil, pgerror.Newf(pgcode.DatatypeMismatch,
 				"%v types %s and %s cannot be matched", typ, l.Typ, r.Typ)
 		}
-		if l.Hidden != r.Hidden {
-			return nil, fmt.Errorf("%v types cannot be matched", typ)
-		}
 		if l.Typ.Family() == types.UnknownFamily {
 			unionColumns[i].Typ = r.Typ
 		}