[clangd] Send empty diagnostics when a file is closed
The LSP clients cannot know clangd will not send diagnostic updates
for closed files, so we send them an empty list of diagnostics to
avoid showing stale diagnostics for closed files in the UI, e.g. in the
"Problems" pane of VSCode.
Reviewed By: hokein
Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D59757