darwin: expose bootstrapLlvmVersion as argument in the stdenv

This enables the bootstrap stdenv test to specify the actual llvm
of the newly generated build instread of assuming it's the same version
as the current stdenv.

pkgs/stdenv/darwin/default.nix

@@ -4,6 +4,7 @@
 , config
 , overlays
 , crossOverlays ? [ ]
+, bootstrapLlvmVersion ? if localSystem.isAarch64 then "11.1.0" else "7.1.0"
   # Allow passing in bootstrap files directly so we can test the stdenv bootstrap process when changing the bootstrap tools
 , bootstrapFiles ? if localSystem.isAarch64 then
     let
@@ -40,14 +41,11 @@ assert crossSystem == localSystem;
 let
   inherit (localSystem) system;
 
-  # Bootstrap version needs to be known to reference headers included in the bootstrap tools
-  bootstrapLlvmVersion = if localSystem.isAarch64 then "11.1.0" else "7.1.0";
-
   useAppleSDKLibs = localSystem.isAarch64;
   haveKRB5 = localSystem.isx86_64;
 
   # final toolchain is injected into llvmPackages_${finalLlvmVersion}
-  finalLlvmVersion = if localSystem.isAarch64 then "11" else "7";
+  finalLlvmVersion = lib.versions.major bootstrapLlvmVersion;
   finalLlvmPackages = "llvmPackages_${finalLlvmVersion}";
 
   commonImpureHostDeps = [

pkgs/stdenv/darwin/make-bootstrap-tools.nix

@@ -254,6 +254,8 @@ in rec {
     '';
   };
 
+  bootstrapLlvmVersion = llvmPackages.llvm.version;
+
   bootstrapFiles = {
     sh      = "${build}/on-server/sh";
     bzip2   = "${build}/on-server/bzip2";
@@ -408,7 +410,7 @@ in rec {
     system = if crossSystem != null then crossSystem else system;
 
     stdenvStages = args: let
-        args' = args // { inherit bootstrapFiles; };
+        args' = args // { inherit bootstrapLlvmVersion bootstrapFiles; };
       in (import (test-pkgspath + "/pkgs/stdenv/darwin") args').stagesDarwin;
   };
 }