[clang-refactor] Get rid of OccurrencesFinder in RenamingAction, NFC

Summary:
The OccurrencesFinder is only used in RenameOccurrences to find symbol
occurrences, there is no need to inherit RefactoringRule.

Replace it with a single utility function to avoid code misleading.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D39796

llvm-svn: 317696