[clangd] Set workspace root when initializing ClangdServer, disallow mutation.

Summary:
Rename instance variable to WorkspaceRoot to match what we call it internally.

Add fixme to set it automatically. Don't do it yet, clients have assumptions
that the constructor won't access the FS.

Don't second-guess the provided root.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D53404

llvm-svn: 344787