[libclang] Introduce a CXCursor_ObjCSelfExpr cursor, which is the equivalent of CXCursor_CXXThisExpr for C++ code.

rdar://13717006

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180127 91177308-0d34-0410-b5e6-96231b3b80d8