Fix driver error while introspecting sequences in SQL Server 2012

It seems the SQL Server drivers don't like SQL Server's special `sql_variant` data type when retrieving data from such columns. Therefore introspecting Sequences in SQL Server 2012 generates the following error:

```php
Fatal error: Unexpected SQL type encountered in calc_string_size. in C:\cygwin\home\Administrator\dev\dbal\lib\Doctrine\DBAL\Driver\SQLSrv\SQLSrvStatement.php on line 226
```

A workaround this limitation is to cast the specific columns to `VARCHAR` in the query.