SELECT - There is no supertype for types DateTime, String

Hello!

I'm faced with an unexpected behaviour which is very similar to #3746.

It's easy to reproduce. The next request:

```sql
SELECT greatest(time, '2021-08-18 10:44:03') AS max_time
FROM
(
SELECT now() AS time
)
```

ends with the error:

```
Received exception from server (version 21.4.6):
Code: 386. DB::Exception: Received from s:9000. DB::Exception: There is no supertype for types DateTime, String because some of them are String/FixedString and some of them are not: While processing greatest(time, '2021-08-18 10:44:03') AS max_time.
```

Possible workaround is to use `toDateTime()` function to explicit cast the string parameter.

```sql
SELECT greatest(time, toDateTime('2021-08-18 10:44:03')) AS max_time
FROM
(
SELECT now() AS time
)

Query id: 091311c2-a7c4-4192-a7b6-d9680c9d1f8d

┌────────────max_time─┐
│ 2021-08-18 10:49:03 │
└─────────────────────┘

1 rows in set. Elapsed: 0.006 sec.
```

The server version is `21.4.6.55`.