llvmPackages: still use 3.7.x on Darwin (#20461)

I'm sorry, I forgot that.

pkgs/top-level/all-packages.nix

@@ -5062,7 +5062,8 @@ in
   llvm_35 = llvmPackages_35.llvm;
   llvm_34 = llvmPackages_34.llvm;
 
-  llvmPackages = recurseIntoAttrs llvmPackages_39;
+  llvmPackages = recurseIntoAttrs
+    (if stdenv.isDarwin then llvmPackages_37 else llvmPackages_39);
 
   llvmPackagesSelf = llvmPackages_34.override {
     stdenv = libcxxStdenv;