Summary:
In order to remove the config pointer from nodes, we have to keep track of whether the node is using web defaults.
This information fits into one bit that we can place in padding (i.e. no extra memory needed).
This allows us to get rid of config usage withing `YGNode` with some exceptions:
- `iterChildrenAfterCloningIfNeeded` -- this function will simply receive the configuration, or the cloning callback.
- `setAndPropogateUseLegacyFlag` -- will be removed in D15316863
- in `YGNode::reset` -- will go away utomatically once we remove the config pointer
Reviewed By: SidharthGuglani
Differential Revision: D15391536
fbshipit-source-id: 0fa0d0805c6862bd741fe4a7d9b637ed534f56a4
1938792517