[WIP] darwin: update bootstrap-tools to use llvm_9

[LnL7]

This is in preparation for a llvm 7 -> 9 update, bootstrapping fails with our current bootstrap-tools. But this means our bootstrap tarball test won't match the default version until this is complete. @copumpkin is this the most appropriate order of steps to get a tarball built by hydra that we can put in the tarballs bucket?

Things done

Used the tarball to bootstrap an llvm 9 based stdenv and a bunch of base packages like python, etc.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @NixOS/darwin-maintainers

[LnL7]

Part of #85151.