Whatever change has necessitated
https://github.com/NixOS/nixpkgs/pull/122044, it also broke clangd --
<clang-wrapper>/resource-root/include is no longer automagically
searched for includes, which kills pretty much any indexing since that
directory contains vital stuff like stddef.h etc.
Fix by appending the directory to CPATH & CPLUS_INCLUDE_PATH in the
clangd wrapper.