[libclang 5/8] Add support for ObjC attributes without args

Summary:
This adds support to libclang for identifying ObjC related attributes that don't take arguments.

All attributes but NSObject and NSConsumed are tested.

Reviewers: yvvan, jbcoe

Reviewed By: yvvan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49127

llvm-svn: 338813