This wrapper derivation (which assumes that the depot is available at
~/depot) can be used to actually get clangd working with
//third_party/nix.
In my setup I can launch this with M-x eglot, followed by
env
CLANGD_FLAGS='--compile-commands-dir=/home/tazjin/projects/nix-build'
nix-shell -A third_party.nix --run 'nix-clangd' /home/tazjin/depot