nix: fix prebuilt LLVM in dev shell

Add the packages required for prebuilt LLVM to LD_LIBRARY_PATH.

Also, switch from `stdenvNoCC` to a proper clang/llvm stdenv.

Author: rkoser-nv

flake.nix

@@ -14,12 +14,14 @@
       system:
       let
         pkgs = import nixpkgs { inherit system; };
+        llvmPackages = pkgs.llvmPackages_21;
+        mkShell = pkgs.mkShell.override { stdenv = llvmPackages.stdenv; };
       in
       {
         # We want to use Clang instead of GCC because it seems to behave better
         # with LLDB, so we use `mkShellNoCC` here instead of `mkShell` because
         # the latter brings in GCC by default on Linux.
-        devShell = pkgs.mkShellNoCC {
+        devShell = mkShell {
           buildInputs = [
             # We must list clangd before the `clang` package to make sure it
             # comes earlier on the `PATH`, and we must get it from the
@@ -39,10 +41,9 @@
               }
             ])
 
-            pkgs.clang
             pkgs.cmake
             pkgs.gersemi
-            pkgs.lldb
+            llvmPackages.lldb
             pkgs.ninja
             pkgs.nixfmt-rfc-style
             pkgs.prettier
@@ -56,6 +57,13 @@
             # other than NixOS may require the use of nixGL:
             # https://github.com/nix-community/nixGL
             pkgs.vulkan-loader
+            # Needed for the prebuilt LLVM
+            pkgs.libz
+            pkgs.zstd
+            # Despite requiring this packages (slang) to be built with Clang,
+            # the prebuilt libslang-llvm.so is actually linked against GCC's
+            # libstdc++.so.6
+            pkgs.stdenv.cc.cc.lib
           ];
         };
       }