os/linux: add more Xcode stubs.

Saw these on a random user report.

Library/Homebrew/os/linux.rb

@@ -68,6 +68,10 @@ module OS
       def version
         Version::NULL
       end
+
+      def installed?
+        false
+      end
     end
 
     module CLT
@@ -76,6 +80,10 @@ module OS
       def version
         Version::NULL
       end
+
+      def installed?
+        false
+      end
     end
   end
   # rubocop:enable Style/Documentation