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/Utils.cpp

@@ -55,7 +55,7 @@ float YGFloatSanitize(const float& val) {
 }
 
 float YGUnwrapFloatOptional(const YGFloatOptional& op) {
-  return op.isUndefined ? YGUndefined : op.value;
+  return op.isUndefined() ? YGUndefined : op.getValue();
 }
 
 bool YGFloatOptionalFloatEquals(