[SPARK-14910][SQL] Native DDL Command Support for Describe Function in Non-identifier Format

#### What changes were proposed in this pull request?
The existing `Describe Function` only support the function name in `identifier`. This is different from what Hive behaves. That is why many test cases `udf_abc` in `HiveCompatibilitySuite` are not using our native DDL support. For example,
- udf_not.q
- udf_bitwise_not.q

This PR is to resolve the issues. Now, we can support the command of `Describe Function` whose function names are in the following format:
- `qualifiedName` (e.g., `db.func1`)
- `STRING` (e.g., `'func1'`)
- `comparisonOperator` (e.g,. `<`)
- `arithmeticOperator` (e.g., `+`)
- `predicateOperator` (e.g., `or`)

Note, before this PR, we only have a native command support when the function name is in the format of `qualifiedName`.
#### How was this patch tested?
Added test cases in `DDLSuite.scala`. Also manually verified all the related test cases in `HiveCompatibilitySuite` passed.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #12679 from gatorsmile/descFunction.