llvmPackages.clang-unwrapped: enable debug info

This increases the memory required to link by quite a bit, but it
should be fine for Hydra, and people building themselves with
insufficient RAM can either disable it, or just decrease --cores.

pkgs/development/compilers/llvm/common/clang/default.nix

@@ -201,6 +201,8 @@ stdenv.mkDerivation (
       "python"
     ];
 
+    separateDebugInfo = true;
+
     postInstall =
       ''
         ln -sv $out/bin/clang $out/bin/cpp