Restore check for broken xcode-select path

How did this get removed? Had a bug today where user had this but no doctor check so I was confused!

Library/Homebrew/cmd/doctor.rb

@@ -392,8 +392,14 @@ end
 
 def check_xcode_select_path
   # with the advent of CLT-only support, we don't need xcode-select
-  return if MacOS::CLT.installed?
-  unless File.file? "#{MacOS::Xcode.folder}/usr/bin/xcodebuild" and not MacOS::Xcode.bad_xcode_select_path?
+
+  if MacOS::Xcode.bad_xcode_select_path?
+    <<-EOS.undent
+      Your xcode-select path is set to /
+      You must unset it or builds will hang:
+        sudo rm /usr/share/xcode-select/xcode_dir_link
+    EOS
+  elsif not MacOS::CLT.installed? and not File.file? "#{MacOS::Xcode.folder}/usr/bin/xcodebuild"
     path = MacOS.app_with_bundle_id(MacOS::Xcode::V4_BUNDLE_ID) || MacOS.app_with_bundle_id(MacOS::Xcode::V3_BUNDLE_ID)
     path = '/Developer' if path.nil? or not path.directory?
     <<-EOS.undent