[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.