[SPARK-4674] Refactor getCallSite

The current version of `getCallSite` visits the collection of `StackTraceElement` twice. However, it is unnecessary since we can perform our work with a single visit. We also do not need to keep filtered `StackTraceElement`.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #3532 from viirya/refactor_getCallSite and squashes the following commits:

62aa124 [Liang-Chi Hsieh] Fix style.
e741017 [Liang-Chi Hsieh] Refactor getCallSite.