[SPARK-14177][SQL] Native Parsing for DDL Command "Describe Database" and "Alter Database"

#### What changes were proposed in this pull request?

This PR is to provide native parsing support for two DDL commands:  ```Describe Database``` and ```Alter Database Set Properties```

Based on the Hive DDL document:
https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL

##### 1. ALTER DATABASE
**Syntax:**
```SQL
ALTER (DATABASE|SCHEMA) database_name SET DBPROPERTIES (property_name=property_value, ...)
```
 - `ALTER DATABASE` is to add new (key, value) pairs into `DBPROPERTIES`

##### 2. DESCRIBE DATABASE
**Syntax:**
```SQL
DESCRIBE DATABASE [EXTENDED] db_name
```
 - `DESCRIBE DATABASE` shows the name of the database, its comment (if one has been set), and its root location on the filesystem. When `extended` is true, it also shows the database's properties

#### How was this patch tested?
Added the related test cases to `DDLCommandSuite`

Author: gatorsmile <gatorsmile@gmail.com>
Author: xiaoli <lixiao1983@gmail.com>
Author: Xiao Li <xiaoli@Xiaos-MacBook-Pro.local>

This patch had conflicts when merged, resolved by
Committer: Yin Huai <yhuai@databricks.com>

Closes #11977 from gatorsmile/parseAlterDatabase.