This makes clang somewhat forward-compatible with new CUDA releases without
having to patch it for every minor release without adding any new functionality.
If an unknown version is found, clang issues a warning (can be disabled with -Wno-cuda-unknown-version)
and assumes that it has detected the latest known version. CUDA releases are usually supersets
of older ones feature-wise, so it should be sufficient to keep released clang versions
working with minor CUDA updates without having to upgrade clang, too.