Get class name with or without namespace

Summary:

• CodeClass::getClassName allows to get class name with namespace
• CodeClass::getShortClassName returns only the class name

Test Plan:
This is used in Zed for XmlDocument serialization:

$key = CodeClass::from($value)->getShortClassName();

Unit tests provided.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3215