Moved YGFloatOptional from C struct to C++ struct

Summary:
Earlier `YGfloatOptional` was plain struct with no privacy around the variables. This diff adds privacy and also enforces checks when one tries to access value of an undefined `YGFloatOptional`

This diff also adds a behaviour in which when a value of an undefined YGFloatOptional is accessed, it will normally terminate(Several cleanup steps are performed).

Reviewed By: emilsjolander

Differential Revision: D7288555

fbshipit-source-id: f61cc92c8fd0d48d2fc1f4d0e6fcef155f19ff8a

yoga/YGFloatOptional.h

@@ -0,0 +1,26 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+struct YGFloatOptional {
+ private:
+  float value_;
+  bool isUndefined_;
+
+ public:
+  YGFloatOptional(const float& value);
+  YGFloatOptional();
+
+  // Program will terminate if the value of an undefined is accessed. Please
+  // make sure to check if the optional is defined before calling this function.
+  // To check if float optional is defined, use `isUndefined()`.
+  float getValue() const;
+
+  // Sets the value of float optional, and thus isUndefined is assigned false.
+  void setValue(const float& val);
+
+  bool isUndefined() const;
+};