[libclang 1/8] Add support for ObjCObjectType

Summary: This patch adds support to the clang-c API for identifying ObjCObjects in CXTypes, enumerating type args and protocols on ObjCObjectTypes, and retrieving the base type of ObjCObjectTypes. Currently only ObjCInterfaceTypes are exposed, which do not have type args or protocols.

Reviewers: yvvan, jbcoe

Reviewed By: yvvan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49063

llvm-svn: 338804