[clangd] Improve ObjC property handling in SelectionTree.

Fixes https://github.com/clangd/clangd/issues/233

Reviewers: dgoldman

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72634