os/mac/diagnostic: detect Bitdefender.

This has caused us multiple hard-to-diagnose issues so let's complain
about it in `brew doctor`.

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -297,6 +297,19 @@ module Homebrew
         end
       end
 
+      def check_for_bitdefender
+        if !Pathname("/Library/Bitdefender/AVP/EndpointSecurityforMac.app").exist? &&
+           !Pathname("/Library/Bitdefender/AVP/BDLDaemon").exist?
+          return
+        end
+
+        <<~EOS
+          You have installed Bitdefender. The "Traffic Scan" option interferes with
+          Homebrew's ability to download packages. See:
+            https://github.com/Homebrew/brew/issues/5558
+        EOS
+      end
+
       def check_for_multiple_volumes
         return unless HOMEBREW_CELLAR.exist?