[clangd] Enable clangd on Objective-C in VSCode

Summary: Thanks to Andreas Ostermeyer for raising this on the mailing list.

Reviewers: hokein

Reviewed By: hokein

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57813

llvm-svn: 353295