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/debug/Log.cpp

@@ -22,10 +22,11 @@ void vlog(
     void* context,
     const char* format,
     va_list args) {
-  const yoga::Config* logConfig =
-      config != nullptr ? config : resolveRef(YGConfigGetDefault());
-
-  logConfig->log(const_cast<yoga::Node*>(node), level, context, format, args);
+  if (config == nullptr) {
+    getDefaultLogger()(nullptr, node, level, format, args);
+  } else {
+    config->log(node, level, context, format, args);
+  }
 }
 } // namespace