## What changes were proposed in this pull request?
Spark SQL only has `StringType`, when reading hive table with varchar column, we will read that column as `StringType`. However, we still need to use varchar `ObjectInspector` to read varchar column in hive table, which means we need to know the actual column type at hive side.
In Spark 2.1, after https://github.com/apache/spark/pull/14363 , we parse hive type string to catalyst type, which means the actual column type at hive side is erased. Then we may use string `ObjectInspector` to read varchar column and fail.
This PR keeps the original hive column type string in the metadata of `StructField`, and use it when we convert it to a hive column.
## How was this patch tested?
newly added regression test
Author: Wenchen Fan <wenchen@databricks.com>
Closes#16060 from cloud-fan/varchar.
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