mesa's llvm: 7 -> 9

This is needed to support some new GPUs: #68809

pkgs/top-level/all-packages.nix

···
   };
 
   mesa = callPackage ../development/libraries/mesa {
-    # 7 is the default, but only on Linux, so keep this for now
-    llvmPackages = llvmPackages_7;
+    llvmPackages = llvmPackages_9;
     inherit (darwin.apple_sdk.frameworks) OpenGL;
     inherit (darwin.apple_sdk.libs) Xplugin;
   };