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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317696 91177308-0d34-0410-b5e6-96231b3b80d8