[clangd] Set up machinery for gtests of ClangdLSPServer.

Summary:
This is going to be needed to test e.g. diagnostics regeneration on
didSave where files changed on disk. Coordinating such changes is too
hard in lit tests.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77766