[Backport staging-25.11] llvmPackages.clang-tools: add support for -cxx-isystem#462884

wolfgangwalther merged 1 commit intostaging-25.11NixOS/nixpkgs:staging-25.11from

backport-462747-to-staging-25.11NixOS/nixpkgs:backport-462747-to-staging-25.11Copy head branch name to clipboard