this adds libc++ to the LLVM cross, giving us access to the full
Nixpkgs set. This requires 4 stages of wrapped compilers:

• Clang with no libraries
• Clang with just compiler-rt
• Clang with Libc, and compiler-rt
• Clang with Libc++, Libc, and compiler-rt

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nix-review --run "nix-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

Test:

with import ./default.nix {
  crossSystem = (import ./lib).systems.examples.aarch64-multiplatform-musl // {useLLVM = true;};
};
let
cpp = builtins.toFile "test.c++" ''
#include <iostream>

int main() {
  std::cout << "Hello world!" << std::endl;
}
'';
testdrv = { callPackage, stdenv, llvmPackages_8 }: stdenv.mkDerivation
    {
        name = "cross-test";
        version = "0.1";
        unpackPhase = "true";
        buildInputs = [llvmPackages_8.libunwind];
        buildPhase = ''
            $CXX -lunwind -o test ${cpp}
        '';
        installPhase = ''
            mkdir -p $out/bin
            cp test $out/bin/test
            ${stdenv.hostPlatform.emulator buildPackages} $out/bin/test
        '';
    };
in callPackage testdrv {}