Move out YGValue

Summary:
@public

Creates a single header file for `YGValue`. This is in preparation of a more compact representation of `YGValue` within `YGStyle`.

Also fixes the incorrect definition of NAN.

Reviewed By: SidharthGuglani

Differential Revision: D13439602

fbshipit-source-id: 68eef2c391b6c9810f3c995b86fff7204ebe6511

yoga/YGValue.cpp

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#include "YGValue.h"
+
+const YGValue YGValueZero = {0, YGUnitPoint};
+const YGValue YGValueUndefined = {YGUndefined, YGUnitUndefined};
+const YGValue YGValueAuto = {YGUndefined, YGUnitAuto};
+
+bool operator==(const YGValue& lhs, const YGValue& rhs) {
+  if (lhs.unit != rhs.unit) {
+    return false;
+  }
+
+  switch (lhs.unit) {
+    case YGUnitUndefined:
+    case YGUnitAuto:
+      return true;
+    default:
+      return lhs.value == rhs.value;
+  }
+}
+
+bool operator!=(const YGValue& lhs, const YGValue& rhs) {
+  return !(lhs == rhs);
+}