add extra acronyms for objc property names

Summary: This is to support general acronyms in Objective-C like 2G/3G/4G/... and coordinates X, Y, Z and W.

Reviewers: benhamilton

Reviewed By: benhamilton

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D45750

llvm-svn: 330286