add check to avoid throwing objc exception according to Google Objective-C guide

Summary:
This is a small check to avoid throwing objc exceptions.
In specific it will detect the usage of @throw statement and throw warning.

Reviewers: hokein, benhamilton

Reviewed By: hokein, benhamilton

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D40058

llvm-svn: 318366