mesa: Switch from LLVM 9 to llvmPackages_latest

The radeonsi and radv (without ACO) drivers requires LLVM 11 for
GFX 10.3 [0],[1]. E.g. Arch Linux also compiles Mesa with LLVM 11 so
this should be fine.

[0]: https://docs.mesa3d.org/relnotes/20.2.0.html
[1]: https://github.com/NixOS/nixpkgs/pull/105817#issuecomment-738888102

pkgs/top-level/all-packages.nix

@@ -14962,7 +14962,7 @@ in
   libGLU = mesa_glu;
 
   mesa = callPackage ../development/libraries/mesa {
-    llvmPackages = llvmPackages_9;
+    llvmPackages = llvmPackages_latest;
     inherit (darwin.apple_sdk.frameworks) OpenGL;
     inherit (darwin.apple_sdk.libs) Xplugin;
   };