Mark enum with [Flags] where they are used as flags

Summary:
Marks enum with ```[Flags]``` attribute, where they are used as flags.
Closes https://github.com/facebook/yoga/pull/515

Reviewed By: emilsjolander

Differential Revision: D4921807

Pulled By: splhack

fbshipit-source-id: f056189b76ee6e6c042417a7998bfd20a610c27f

enums.py

@@ -223,6 +223,8 @@ for name, values in sorted(ENUMS.items()):
     with open(root + '/csharp/Facebook.Yoga/Yoga%s.cs' % name, 'w') as f:
         f.write(LICENSE)
         f.write('namespace Facebook.Yoga\n{\n')
+        if isinstance(next(iter(values or []), None), tuple):
+            f.write('    [System.Flags]\n')
         f.write('    public enum Yoga%s\n    {\n' % name)
         for value in values:
             if isinstance(value, tuple):