.ForXxx doesn't work with From()

If I want, for example, to use a different table name in different databases:

```csharp
var query = new Query()
    .Select("Id", "Title")
    .ForPostgreSql(q => q.From("Foo"))
    .ForSqlServer(q => q.From("Bar"));
```
This will produce `FROM Foo` (with proper escaping) regardless of which compiler I use -- even if I use a compiler other than Postgres or SQL Server.