[clangd] Use buffered llvm::errs() in the clangd binary.

Summary: Unbuffered stream can cause significant (non-deterministic) latency for the logger.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D51349

llvm-svn: 340822