[SPARK-18497][SS] Make ForeachSink support watermark

## What changes were proposed in this pull request?

The issue in ForeachSink is the new created DataSet still uses the old QueryExecution. When `foreachPartition` is called, `QueryExecution.toString` will be called and then fail because it doesn't know how to plan EventTimeWatermark.

This PR just replaces the QueryExecution with IncrementalExecution to fix the issue.

## How was this patch tested?

`test("foreach with watermark")`.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15934 from zsxwing/SPARK-18497.

(cherry picked from commit 2a40de40)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>