to_string(a) requires a to be in the select list

Nobody has claimed this yet.

`to_string()` works only if the `` is in the select list too, otherwise it returns an empty value:

```
mysql> drop table if exists t; create table t(a bool) engine='columnar'; insert into t(id, a) values(1, 1); select to_string(a) from t; select a, to_string(a) from t;
--------------
drop table if exists t
--------------

Query OK, 0 rows affected (0.01 sec)

--------------
create table t(a bool) engine='columnar'
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
insert into t(id, a) values(1, 1)
--------------

Query OK, 1 row affected (0.00 sec)

--------------
select to_string(a) from t
--------------

--------------
| to_string(a) |
--------------
| |
--------------
1 row in set (0.00 sec)

--------------
select a, to_string(a) from t
--------------

------ --------------
| a | to_string(a) |
------ --------------
| 1 | 1 |
------ --------------
1 row in set (0.00 sec)
```

Start by reproducing the issue with the SQL sequence in the report, comparing `select to_string(a) from t` with `select a, to_string(a) from t` on a columnar table. Trace the C++ handling of `to_string(bool)` and identify why the expression depends on the select list. Done means the first query returns the boolean value as a string without selecting `a` separately, with a regression test covering both queries.

Written by the indexing model from the issue text.