clang: fix install on Darwin

[strager]

Motivation for this change

As of Nix 2.0, building the user-environment package on macOS (Darwin) fails because LLVMgold.so is a broken symlink. Fix the issue by not creating the symlink in the first place, since it wouldn't be used on Darwin anyway.

Requests for comments

It seems that Nix 2.0's new buildenv implementation breaks with broken symlinks. Where can I find the code for the new buildenv implementation? The buildenv source in this repository (NixOS/nixpkgs) looks like a Perl script, not a compiled binary which I have in /nix/store/xmi4fylvx4qc79ji9v5q3zfy9vfdy4sv-nix-2.0/libexec/nix/buildenv.

I think these changes will cause full rebuilds. Should they go on master or staging?

Should I bother with other versions of Clang (e.g. 4.0.1 in pkgs/development/compilers/llvm/4/default.nix)? How can I install those with nix-env -i?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platforms: nix-env -i --no-build-output --max-jobs 3 --cores 8 -f . clang
  • 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md

Things to do

• Test more thoroughly
• Update other versions of Clang
• Figure out if LLVM's Gold LTO plugin is supposed to be built on Darwin
• Find the change in Nix 2.0 or buildenv which caused the broken symlink to break the build

[matthewbauer]

Looks good! LLVM gold is only provided on Linux I think. LTO on Darwin is still broken anyway (#19098).

Note that the base for this should be "staging". I also want to cherry-pick 84088930985e6d8dd805661596690453d4ee3f03 (probably will create a PR) so please ping me after merging this.

[strager]

I'm unfamiliar with nixpkgs' process.

• Why did you merge this into staging before I addressed my own to-do items (like testing on Linux)?
• Is there any work I need to do now?
• What is commit 7b73c7f and how is it different from 2505aa5?

[matthewbauer]

• This is a pretty straightforward change and I also wanted to apply 9ee69c2 which modifies clang.
• We should be good. Staging will pick up the commit soon, and we'll verify that no regressions are introduced in Hydra: https://hydra.nixos.org/jobset/nixpkgs/staging. I'll follow the status there and we can always revert it if there are problems.
• 7b73c7f is just applying the change to older LLVMs.

[LnL7]

@strager The checkboxes are there as information for reviewers, you're not required to complete all of those.