Summary: Add AST matchers for Objective-C @throw, @try, @catch and @finally.

Reviewers: aaron.ballman, malcolm.parsons, alexshap, compnerd

Reviewed By: aaron.ballman

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D39940

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317992 91177308-0d34-0410-b5e6-96231b3b80d8