[SPARK-12841][SQL] fix cast in filter

In SPARK-10743 we wrap cast with `UnresolvedAlias` to give `Cast` a better alias if possible. However, for cases like `filter`, the `UnresolvedAlias` can't be resolved and actually we don't need a better alias for this case.  This PR move the cast wrapping logic to `Column.named` so that we will only do it when we need a alias name.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10781 from cloud-fan/bug.