[clangd] Allow diagnostics to be suppressed with configuration

This has been specifically requested:
  https://github.com/clangd/vscode-clangd/issues/114
and various issues can be addressed with this as a workaround, e.g.:
  https://github.com/clangd/clangd/issues/662

Differential Revision: https://reviews.llvm.org/D95349