pkgs.overrideLibcxx

The following is the current implementation of this function.

overrideLibcxx =
    stdenv:
    assert stdenv.cc.libcxx != null;
    assert pkgs.stdenv.cc.libcxx != null;
    # only unified libcxx / libcxxabi stdenv's are supported
    assert lib.versionAtLeast pkgs.stdenv.cc.libcxx.version "12";
    assert lib.versionAtLeast stdenv.cc.libcxx.version "12";
    let
      llvmLibcxxVersion = lib.getVersion llvmLibcxx;

      stdenvLibcxx = pkgs.stdenv.cc.libcxx;
      llvmLibcxx = stdenv.cc.libcxx;

      libcxx =
        pkgs.runCommand "${stdenvLibcxx.name}-${llvmLibcxxVersion}"
          {
            outputs = [
              "out"
              "dev"
            ];
            isLLVM = true;
          }
          ''
            mkdir -p "$dev/nix-support"
            ln -s '${stdenvLibcxx}' "$out"
            echo '${stdenvLibcxx}' > "$dev/nix-support/propagated-build-inputs"
            ln -s '${lib.getDev llvmLibcxx}/include' "$dev/include"
          '';
    in
    overrideCC stdenv (
      stdenv.cc.override {
        inherit libcxx;
        extraPackages = [
          pkgs.buildPackages.targetPackages."llvmPackages_${lib.versions.major llvmLibcxxVersion}".compiler-rt
        ];
      }
    );