Fix the issue that not recognizing single acronym with prefix as ObjC property name.

Summary: This will make clang-tidy accept property names like xyz_URL (URL is a common acronym).

Reviewers: benhamilton, hokein

Reviewed By: benhamilton

Subscribers: jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D53955

llvm-svn: 345858