Fabric: Change naming scheme of ShadowNodeFragment placeholders (#23437)

Summary:
This pull request implements some suggestions made in #23414, namely:

* Rename `ShadowNodeFragment::nullSharedProps()` (and family) to `ShadowNodeFragment::propsPlaceholder()`;
* Introduce similar functions for the rest of members (tag and rootTag);

Later pull requests to remove designated initializers from Fabric (for MSVC support) will use these

I'd like to do the rename of `rootTag` to `surfaceId` as well, but would make sense to do in a separate diff as that would result in a great deal of changes.

[General] [Changed] - Changed naming scheme of `ShadowNodeFragment` placeholders, and added placeholders for remaining members
Pull Request resolved: https://github.com/facebook/react-native/pull/23437

Reviewed By: mdvacca

Differential Revision: D14072007

Pulled By: shergin

fbshipit-source-id: d5bb10a82c16d45955f417a49b503f215c4bc521

ReactCommon/fabric/core/shadownode/ShadowNodeFragment.cpp

@@ -10,22 +10,30 @@
 namespace facebook {
 namespace react {
 
-SharedProps &ShadowNodeFragment::nullSharedProps() {
+Tag ShadowNodeFragment::tagPlaceholder() {
+  return 0;
+}
+
+Tag ShadowNodeFragment::surfaceIdPlaceholder() {
+  return 0;
+}
+
+SharedProps &ShadowNodeFragment::propsPlaceholder() {
   static auto &instance = *new SharedProps();
   return instance;
 }
 
-SharedEventEmitter &ShadowNodeFragment::nullSharedEventEmitter() {
+SharedEventEmitter &ShadowNodeFragment::eventEmitterPlaceholder() {
   static auto &instance = *new SharedEventEmitter();
   return instance;
 }
 
-SharedShadowNodeSharedList &ShadowNodeFragment::nullSharedChildren() {
+SharedShadowNodeSharedList &ShadowNodeFragment::childrenPlaceholder() {
   static auto &instance = *new SharedShadowNodeSharedList();
   return instance;
 }
 
-SharedLocalData &ShadowNodeFragment::nullLocalData() {
+SharedLocalData &ShadowNodeFragment::localDataPlaceholder() {
   static auto &instance = *new SharedLocalData();
   return instance;
 }

ReactCommon/fabric/core/shadownode/ShadowNodeFragment.h

@@ -26,15 +26,17 @@ namespace react {
 struct ShadowNodeFragment {
   Tag tag = 0;
   Tag rootTag = 0;
-  const SharedProps &props = nullSharedProps();
-  const SharedEventEmitter &eventEmitter = nullSharedEventEmitter();
-  const SharedShadowNodeSharedList &children = nullSharedChildren();
-  const SharedLocalData &localData = nullLocalData();
+  const SharedProps &props = propsPlaceholder();
+  const SharedEventEmitter &eventEmitter = eventEmitterPlaceholder();
+  const SharedShadowNodeSharedList &children = childrenPlaceholder();
+  const SharedLocalData &localData = localDataPlaceholder();
 
-  static SharedProps &nullSharedProps();
-  static SharedEventEmitter &nullSharedEventEmitter();
-  static SharedShadowNodeSharedList &nullSharedChildren();
-  static SharedLocalData &nullLocalData();
+  static Tag tagPlaceholder();
+  static Tag surfaceIdPlaceholder();
+  static SharedProps &propsPlaceholder();
+  static SharedEventEmitter &eventEmitterPlaceholder();
+  static SharedShadowNodeSharedList &childrenPlaceholder();
+  static SharedLocalData &localDataPlaceholder();
 };
 
 } // namespace react

ReactCommon/fabric/uimanager/UIManager.cpp

@@ -47,7 +47,7 @@ SharedShadowNode UIManager::cloneNode(
       {
           .props = rawProps ? componentDescriptor.cloneProps(
                                   shadowNode->getProps(), *rawProps)
-                            : ShadowNodeFragment::nullSharedProps(),
+                            : ShadowNodeFragment::propsPlaceholder(),
           .children = children,
       });