[clangd] Show FileStatus in vscode-clangd.

Summary:
The file status will be shown in the status bar.
Depends on D55363.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D55374

llvm-svn: 350210