stdenv/darwin: assemble full clang toolchain

[thefloweringash]

Adapted from main expression for clang 7.

Motivation for this change

Second part of #94426, and must only be merged when updating bootstrap tools to include #94426. See that ticket for the main context. This integrates compiler-rt into the toolchain, both from the re-added bootstrap level and for the higher level stdenvs.

I'm not confident about the composition of the stages or if compiler-rt is rebuilt at the appropriate times. The idea is to preserve the bootstrap version until llvm is available to build a new one.

cc @matthewbauer and @LnL7

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.

[thefloweringash]

The current state of this now is able to build perlPackages.LocaleGettext from an updated bootstrap tools. I put together a simple analysis for the structure of the stdenv and verified that this doesn't not change the structure in any interesting way. Since llvmPackages_7.compiler-rt depends on llvm, it is delayed until the same stage as the first llvm rebuild by copying it up from bootstrap tools until then.

The only remaining mystery is if libcxxabi is required. Including it in the same way as the main clang expression causes libcxx to reference two different libcxxabi packages.

[thefloweringash]

There are two commits here:

• the first is the minimal set of changes to produce a functional stdenv after updating the bootstrap tools
• the second is to try to improve the correctness of the bootstrapping change, and is not complete (looking for feedback)

The main change in the second commit is to match the toolchain wrapper in llvm/7/default.nix and include libcxxabi in extraPackages. Testing with only the first commit shows that it works without this, so maybe it's not important. In order to include libcxxabi, I had to be careful when building it to avoid a reference to the bootstrap version. For this I took the bootstrapping logic from llvm/7/default.nix [1], [2]. This existing bootstrapping logic is driven by stdenv.hostPlatform.useLLVM, which I initially expected to be true for Darwin as it is an llvm/clang based stdenv, but it appears to be unrelated (used for wasi cross compilation).

I've spent a bit of time inspecting the darwin stdenv. I noticed that the bootstrap tools don't appear to be swapped out incrementally through the stages as they are in linux. The second commit also includes an attempt to make this more well behaved.

[thefloweringash]

This has very similar effects to stdenv.hostPlatform.useLLVM, in supporting bootstraping an llvm/clang based toolchain. Though useLLVM is only defined in versions 8, 9, 10 and 11. We should find a consistent approach to this. If possible I'd like to remove useLLVM from llvm/*/libc++abi.nix for something reusable that describes its effects.

[thefloweringash]

This has been subsumed by #98541.