llvmPackages_git: remove

[Artturin]

There are no maintainers for this as shown by it still being at llvm 15 and it's unnecessary work to apply all changes to it.

It has not been a git version for a long time and often lags behind the latest stable llvm

Here are all the differences between the current base(15) of _git and 15 https://gist.github.com/Artturin/c39265ecaeea523cd7224fc7b7632371

We should do this because it eases (my) work on deduplicating llvmPackages, after more of it is deduplicated then we could reconsider adding this back but for now it's just a burden.

Any user who wants to use a git release can use the gitRelease argument in 13+

Description of changes

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.

Priorities

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[Artturin]

This change is missing from 15 but in #194634

#185766
verify still needed; add a regression passthru test?

appears to be no longer needed; libc++abi.so didn't seem to be linked against older libc++abi on non-darwin anyways (where the default stdenv wasn't LLVM based) and now we unconditionally use the clang from the same package set which has libcxx set to null and thus doesn't try to link in any libc++abi

[Artturin]

This is enableGoldPlugin ? libbfd.hasPluginAPI in 15 because b9a9dfc wasn't ported to git

[Artturin]

I went through all the diffs and there are no changes missing for 15, only changes missing from _git

[Artturin]

llvmPackages_git was requested in #114828

[RossComputerGuy]

Probably shouldn't kill llvmPackages_git. Since LLVM 17, I've been using it as a template for the next LLVM version. Once 18 is merged, I plan on trying to keep llvmPackages_git more up to date and I even am aiming to make everything common.