feat(compdb): write compile_commands.json for clangd

One entry per translation unit with directory, arguments and file, taken from
the compile actions already in the graph.

lib/compdb.ml

@@ -0,0 +1,19 @@
+let render (b : Build.t) =
+  let dir = Sys.getcwd () in
+  let entries =
+    Array.to_list b.g.Graph.nodes
+    |> List.filter (fun (n : Graph.node) -> n.tag = "cc" || n.tag = "c++")
+    |> List.map (fun (n : Graph.node) ->
+           Printf.sprintf
+             "  {\n    \"directory\": %s,\n    \"file\": %s,\n    \"output\": %s,\n    \"arguments\": %s\n  }"
+             (Json.str dir) (Json.str n.label)
+             (Json.str (List.hd n.outs))
+             (Json.list Json.str (Array.to_list n.cmd)))
+  in
+  "[\n" ^ String.concat ",\n" entries ^ "\n]\n"
+
+let write b path =
+  Fs.write path (render b);
+  List.length
+    (List.filter (fun (n : Graph.node) -> n.Graph.tag = "cc" || n.Graph.tag = "c++")
+       (Array.to_list b.Build.g.Graph.nodes))