[SPARK-15199][SQL] Disallow Dropping Build-in Functions

#### What changes were proposed in this pull request?
As Hive and the major RDBMS behave, the built-in functions are not allowed to drop. In the current implementation, users can drop the built-in functions. However, after dropping the built-in functions, users are unable to add them back.

#### How was this patch tested?
Added a test case.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #12975 from gatorsmile/dropBuildInFunction.