clang_15: add nostdlibinc flag

[alyssais]

Description of changes

Port of 44165d3 ("llvmPackages_{14, git}.clang: add nostdlibinc flag") to Clang 15. It was originally thought this wasn't needed, but it is, to fix expressions like the following:

with import ./. {};

llvmPackages_15.libcxxStdenv.mkDerivation {
  name = "libcxx-stdenv-c++-test";

  dontUnpack = true;

  input = ''
    #include <cstdlib>

    int main() {
        std::abort();
        return 0;
    }
  '';
  passAsFile = [ "input" ];

  installPhase = ''
    $CXX -c -o $out -x c++ $inputPath
   '';
}

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.05 Release Notes (or backporting 22.11 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.

[rrbutani]

Thanks for fixing this! I realized here that we actually do need this patch for llvmPackages_15.clang but hadn't gotten around to making the PR 🙂.

Good call on deduplicating the patch files, I'd missed that this one hadn't actually changed across LLVM 13+.

---

It'd be great to have a regression test for this somewhere but it's not super apparent to me where the right place would be (cc-wrapper tests (not yet enabled for llvmPackages_15)? an extra passthru test on libcxxStdenv?) and that doesn't need to block this PR anyways.

[rrbutani]

nit, feel free to ignore: I think it'd be useful to have a comment on this patch file giving a little context about why this is required; maybe an abbreviated version of what's in this comment, something like:

`--gcc-toolchain` causes `libstdc++` paths from `gcc` to be included, breaking `libc++`
`include_next` directives and we can't remove `--gcc-toolchain` when using `libc++`
because we may want to use `libgcc_s` with `libc++`; `--nostdlibinc` lets us retain the
`--gcc-toolchain` flag while keeping it from influencing include paths

Granted, I don't think we'll run into a situation like #194634 again for this patch but even so.

[alyssais]

I think this is adequately covered by my commit message, which I made quite thorough for exactly this reason. IMO it's more useful to have a commit that you can checkout and be guaranteed to reproduce to understand the failure, than a comment that might go out of date.

[rrbutani]

👍 That's fair; it took me a little while to realize why the flag was actually needed but agreed that the test case is more valuable; once you've got it, it's pretty straightforward to figure the rest out.