Handle the case where there are clang-xy binaries but no clang binary

Currently if you install specific clang versions via PPAs/llvm.sh you get a versioned binary for clang, e.g. clang-17, but no unversioned clang binary. This causes the following exception:

FileNotFoundError: [Errno 2] No such file or directory: PosixPath('clang')

To work around this issue the clang and llvm packages & dependencies have to be installed (requiring ~300 MB) even though they won't be used.