Ensure clangd finds headers without manual trigger of compile_commands.json generation

[jacg]

Currently, LSP doesn't work properly out of the box, because clangd can't find headers until compile_commands.json is generated. This happens when the project is built, but it is remove again when the project is just cleaned.

We should

1. add something to the shell hooks that ensures the required clangd-helper data are automatically present
2. ensure that just clean does not remove them

This looks relevant: https://discourse.nixos.org/t/get-clangd-to-find-standard-headers-in-nix-shell/11268/13

[jacg]

It seems that this is now taken care of in Nix itself:

• https://github.com/danielbarter/mini_compile_commands
• cc-wrapper: adding a cc-wrapper-hook to the cc-wrapper NixOS/nixpkgs#197937
• https://discourse.nixos.org/t/using-nix-infrastructure-to-reliably-generate-compile-commands-json/21961

We just have to figure out how to make use of it.