[clangd] Publish config file errors over LSP

We don't act as a language server for these files (e.g. don't get open/close
notifications for them), but just blindly publish diagnostics for them.

This works reasonably well in coc.nvim and vscode: they show up in the
workspace diagnostic list and when you open the file.
The only update after the file is reparsed, not as you type which is a bit
janky, but seems a lot better than nothing.

Fixes https://github.com/clangd/clangd/issues/614

Differential Revision: https://reviews.llvm.org/D92704