[SPARK-11377] [SQL] withNewChildren should not convert StructType to Seq

[marmbrus]

This is minor, but I ran into while writing Datasets and while it wasn't needed for the final solution, it was super confusing so we should fix it.

Basically we recurse into Seq to see if they have children. This breaks because we don't preserve the original subclass of Seq (and StructType <:< Seq[StructField]). Since a struct can never contain children, lets just not recurse into it.

[cloud-fan]

LGTM

[SparkQA]

Test build #44531 has finished for PR 9334 at commit b5c6f82.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * case class CoGroup[K, Left, Right, R](\n * class CoGroupedIterator(\n * case class CoGroup[K, Left, Right, R](\n

[yhuai]

Merging to master.