Breaking: YGConfigRef related const-correctness fixes (#1371)

Summary:
X-link: https://github.com/facebook/react-native/pull/39374

Pull Request resolved: https://github.com/facebook/yoga/pull/1371

Right now `YGConfigGetDefault` and `YGNodeGetConfig` both return mutable, freeable, configs, which is bad, since the former points to a global singleton config, and the latter usually does too. Mutating this is not thread safe, and it should never be freed.

This change makes these functions return `YGConfigConstRef` to prevent mutation, and also lets us allow `YGConfigNewWithConfig` to accept a const config. If a caller does want to mutate a config (such as to free it), it must be tracked manually.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D49132476

fbshipit-source-id: ac9ce61149e69c6c25cadb99711435b0a5b9f38a

yoga/node/Node.h

@@ -71,7 +71,7 @@ private:
   size_t lineIndex_ = 0;
   Node* owner_ = nullptr;
   std::vector<Node*> children_ = {};
-  Config* config_;
+  const Config* config_;
   std::array<YGValue, 2> resolvedDimensions_ = {
       {YGValueUndefined, YGValueUndefined}};
 
@@ -95,10 +95,8 @@ private:
   Node& operator=(Node&&) = default;
 
 public:
-  Node() : Node{static_cast<Config*>(YGConfigGetDefault())} {
-    flags_.hasNewLayout = true;
-  }
-  explicit Node(Config* config);
+  Node();
+  explicit Node(const Config* config);
   ~Node() = default; // cleanup of owner/children relationships in YGNodeFree
 
   Node(Node&&);
@@ -165,7 +163,7 @@ public:
 
   size_t getChildCount() const { return children_.size(); }
 
-  Config* getConfig() const { return config_; }
+  const Config* getConfig() const { return config_; }
 
   bool isDirty() const { return flags_.isDirty; }