[clangd] add CLANG_ENABLE_CLANGD option to build clangd. Require threads.

Reviewers: gribozavr

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61518

llvm-svn: 360115