chore(3p): upgrade llvm to version 11

Additionally, anchor clang-tools to match the current llvmPackages version.

Change-Id: I27a199cadb55907c3e2deabfd1d14a5b50fc541b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2166
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: glittershark <grfn@gws.fyi>

users/tazjin/dt/default.nix

@@ -1,7 +1,7 @@
 { depot, pkgs, ... }:
 
 let
-  stdenv = with pkgs; overrideCC clangStdenv clang_10;
+  stdenv = with pkgs; overrideCC clangStdenv clang_11;
   abseil_cpp = pkgs.abseil_cpp;
 in stdenv.mkDerivation {
   name = "dt";

users/tazjin/nixos/frog/default.nix

@@ -239,7 +239,7 @@ in depot.lib.fix(self: {
       chromium
       clang-manpages
       clang-tools
-      clang_10
+      clang_11
       curl
       direnv
       dnsutils