chromium{Beta,Dev}: Use LLVM 12

LLVM 12 is required but the build still fails due to other changes that
where introduced in the meantime (and Chromium 90.0.4430.51 introduced
another LLVM failure).

pkgs/applications/networking/browsers/chromium/browser.nix

@@ -89,6 +89,6 @@ mkChromiumDerivation (base: rec {
       then ["aarch64-linux" "x86_64-linux"]
       else [];
     timeout = 172800; # 48 hours (increased from the Hydra default of 10h)
-    broken = elem channel [ "beta" "dev" ]; # Build requires LLVM 12
+    broken = elem channel [ "beta" "dev" ];
   };
 })

pkgs/applications/networking/browsers/chromium/default.nix

@@ -1,5 +1,5 @@
 { newScope, config, stdenv, fetchurl, makeWrapper
-, llvmPackages_11, ed, gnugrep, coreutils, xdg-utils
+, llvmPackages_11, llvmPackages_12, ed, gnugrep, coreutils, xdg-utils
 , glib, gtk3, gnome3, gsettings-desktop-schemas, gn, fetchgit
 , libva ? null
 , pipewire
@@ -39,6 +39,8 @@ let
           inherit (upstream-info.deps.gn) url rev sha256;
         };
       });
+    } // lib.optionalAttrs (lib.versionAtLeast upstream-info.version "90") {
+      llvmPackages = llvmPackages_12;
     });
 
     browser = callPackage ./browser.nix { inherit channel enableWideVine ungoogled; };