[clangd] Send empty diagnostics when a file is closed

Summary:
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.

Fixes PR41217.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59757

llvm-svn: 356880