difftastic: fix build with clang 12+

[reckenrode]

Description of changes

Work around #166205.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[adamcstephens]

linux builds are cached and this fixes darwin.

---

Result of nixpkgs-review pr 268190 run on x86_64-darwin 1

1 package built:

• difftastic

---

Result of nixpkgs-review pr 268190 run on aarch64-darwin 1

1 package built:

• difftastic

[figsoda]

Thanks!

[reckenrode]

I got a notification complaining that this code has been copied and pasted to different derivations. The comment appears to have been deleted, but I wanted to respond and explain.

This change is a workaround that was expected to be needed only short term. The proper fix belongs in either cc-wrapper or ld-wrapper. I did not implement that fix because @toonn is already working on it, and I did not want to step on that work, but these issues needed to be addressed to get the LLVM update through the staging-next cycle.