Add missing clang_13 alias

pkgs/top-level/all-packages.nix

···
     };
   };
 
+  clang_13 = llvmPackages_13.clang;
   clang_12 = llvmPackages_12.clang;
   clang_11 = llvmPackages_11.clang;
   clang_10 = llvmPackages_10.clang;