overlay.nix adds llvmPackages_opencilk to the pinned nixpkgs package set.
The llvm/ subtree is the small LLVM packaging subtree from the upstream
OpenCilk nixpkgs work in NixOS/nixpkgs#494221, including the OpenCilk source
assembly, Cheetah runtime, and wrapper resource-root changes.
This keeps the project off a full nixpkgs fork while preserving the package shape used by the previous fork-based shell:
pkgs.llvmPackages_opencilk.clang
pkgs.llvmPackages_opencilk.llvm