Add new check in google module for Objective-C code to ensure global variables...

Add new check in google module for Objective-C code to ensure global variables follow the naming convention of Google Objective-C Style Guide

Summary:
This is a new checker for objc files in clang-tidy.

The new check finds global variable declarations in Objective-C files that are not follow the pattern of variable names in Google's Objective-C Style Guide.

All the global variables should follow the pattern of "g[A-Z].*" (variables) or "k[A-Z].*" (constants). The check will suggest a variable name that follows the pattern
if it can be inferred from the original name.

Patch by Yan Zhang!

Reviewers: benhamilton, hokein, alexfh

Reviewed By: hokein

Subscribers: Eugene.Zelenko, mgorny

Differential Revision: https://reviews.llvm.org/D39391

llvm-svn: 317552