Remove enum count macros

Summary:
@public
Removes all `YG...Count` macros for enums and replaces them with `facebook::yoga::enums::count<YG...>()`.
This removes the need to manually maintain enum counts.

Same as D13597449, working around a defect in clang < 3.9

Reviewed By: amir-shalem

Differential Revision: D13634622

fbshipit-source-id: 344dc70e167b0caf746fe396cedd200f54e52219

yoga/YGConfig.h

@@ -10,7 +10,8 @@
 #include "Yoga.h"
 
 struct YGConfig {
-  std::array<bool, YGExperimentalFeatureCount> experimentalFeatures = {};
+  std::array<bool, facebook::yoga::enums::count<YGExperimentalFeature>()>
+      experimentalFeatures = {};
   bool useWebDefaults = false;
   bool useLegacyStretchBehaviour = false;
   bool shouldDiffLayoutWithoutLegacyStretchBehaviour = false;