Upgrade Darwin package to build crystal against LLVM 17

[miry]

Homebrew crystal uses llvm 17. The current solution for darwin using llvm 15.

Should it crystal package use the same LLVM version as for homebrew build script?

[straight-shoota]

Yeah, that would be ideal. I think the updates for darwin are usually lagging behind (or happen less frequently) because it's more effort with omnibus. It shouldn't really matter that much which LLVM version is used though. Maybe newer ones offer some small performance gains, but usually it shouldn't be much.