feat(3p/nix): Add a derivation to launch clangd

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

third_party/nix/default.nix

@@ -32,6 +32,7 @@ in stdenv.mkDerivation {
     pkgconfig
     libxml2
     libxslt
+    (import ./clangd.nix pkgs)
   ];
 
  # TODO(tazjin): Some of these might only be required for native inputs
@@ -80,7 +81,6 @@ in stdenv.mkDerivation {
     ln -s $out/bin/nix $out/libexec/nix/build-remote
   '';
 
-  # TODO(tazjin): equivalent of --enable-gc
   # TODO(tazjin): integration test setup?
   # TODO(tazjin): docs generation?
 }