[SPARK-14668][SQL] Move CurrentDatabase to Catalyst

## What changes were proposed in this pull request?

This PR moves `CurrentDatabase` from sql/hive package to sql/catalyst. It also adds the function description, which looks like the following.

```
scala> sqlContext.sql("describe function extended current_database").collect.foreach(println)
[Function: current_database]
[Class: org.apache.spark.sql.execution.command.CurrentDatabase]
[Usage: current_database() - Returns the current database.]
[Extended Usage:
> SELECT current_database()]
```

## How was this patch tested?
Existing tests

Author: Yin Huai <yhuai@databricks.com>

Closes #12424 from yhuai/SPARK-14668.