llvmPackages_12: Improve and extend the meta attributes

[primeos]

Motivation for this change

Not the cleanest solution but at least it's an improvement. This is especially important for the missing licenses and maintainers (see #116646 (comment) and #119541 (comment)).

Things done

• 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 nixpkgs-review --run "nixpkgs-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.

[r-rmcgibbo]

Result of nixpkgs-review pr 119550 at 28887e7 run on aarch64-linux 1

2 packages skipped due to time constraints:

• llvmPackages_12.clang-manpages
• llvmPackages_12.compiler-rt

1 package built successfully:

• llvmPackages_12.libunwind

1 suggestion:

• warning: missing-patch-comment

  Please add a comment on the line above, explaining the purpose of this patch.
  Near pkgs/development/compilers/llvm/12/compiler-rt/default.nix:52:5:

     |
  52 |     ./compiler-rt-codesign.patch # Revert compiler-rt commit that makes codesign mandatory
     |     ^