i only saw old xcode version, not sure how much it matters. clang actually seems to be same/similiar?
```
[nix]⋊> ~/r/e/r/d/file_system on master ⨯ clang --version                                                                                                                                           22:56:37
clang version 5.0.1 (tags/RELEASE_501/final)
Target: x86_64-apple-darwin17.5.0
Thread model: posix
InstalledDir: /nix/store/p85m242dg1zd85ln3cg0agx0q1n0n6h0-clang-5.0.1/bin
[nix]⋊> ~/r/e/r/d/file_system on master ⨯ /usr/bin/clang --version                                                                                                                                  23:17:44
Apple LLVM version 9.1.0 (clang-902.0.39.1)
Target: x86_64-apple-darwin17.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```