mozart: use llvmPackages_8 instead of llvmPackages_5

[RaitoBezarius]

Description of changes

Following issue #44372 logic, we want to get rid of older LLVMs.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.05 Release Notes (or backporting 22.11 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.

[layus]

@RaitoBezarius Thanks so much for having a look at this one. I pushed a commit that simplifies everything ;-). That old llvm has to be llvm4, but the sources we fetch are preprocessed, and do not need that build step anymore.

• Built and tested on x86_64-linux

[RaitoBezarius]

Awesome!