cfb6649fa7
libclc requires a compatible version of spirv-llvm-translator. If we don't have such a version, we get an eval error. This means that OfBorg builds of llvmPackages sets where libclc is unavailable will produce an eval error, which is very confusing. To fix this, restrict the libclc attribute to LLVM versions where it actually exists. Link: https://github.com/NixOS/nixpkgs/pull/414615#issuecomment-3126935312