[SPARK-17732][SQL] ALTER TABLE DROP PARTITION should support comparators
## What changes were proposed in this pull request?
This PR aims to support `comparators`, e.g. '<', '<=', '>', '>=', again in Apache Spark 2.0 for backward compatibility.
**Spark 1.6**
``` scala
scala> sql("CREATE TABLE sales(id INT) PARTITIONED BY (country STRING, quarter STRING)")
res0: org.apache.spark.sql.DataFrame = [result: string]
scala> sql("ALTER TABLE sales DROP PARTITION (country < 'KR')")
res1: org.apache.spark.sql.DataFrame = [result: string]
```
**Spark 2.0**
``` scala
scala> sql("CREATE TABLE sales(id INT) PARTITIONED BY (country STRING, quarter STRING)")
res0: org.apache.spark.sql.DataFrame = []
scala> sql("ALTER TABLE sales DROP PARTITION (country < 'KR')")
org.apache.spark.sql.catalyst.parser.ParseException:
mismatched input '<' expecting {')', ','}(line 1, pos 42)
```
After this PR, it's supported.
## How was this patch tested?
Pass the Jenkins test with a newly added testcase.
Author: Dongjoon Hyun <dongjoon@apache.org>
Closes #15704 from dongjoon-hyun/SPARK-17732-2.
3ce057d001