Expose "unreserved" trait constants that can be mapped per-component

Summary:
Fabric core uses a lot of traits - I am reserving a few more for core usage, and also exposing a few "unreserved" traits.

It is recommended that all custom components that do use traits rely on these constants instead of hard-coding any trait values. That way, in the unlikely event that these values change in the future, it will not break components.

Changelog: [Internal]

Reviewed By: cortinico, RSNara

Differential Revision: D30401743

fbshipit-source-id: fb2e8f5cf33c94e31a0c25a89055acfc4eccf066

ReactCommon/react/renderer/core/ShadowNode.h

@@ -240,5 +240,9 @@ ShadowNodeReferenceT traitCast(ShadowNode const &shadowNode);
 template <typename ShadowNodePointerT>
 ShadowNodePointerT traitCast(ShadowNode const *shadowNode);
 
+template <typename ShadowNodePointerT>
+std::shared_ptr<ShadowNodePointerT const> traitCast(
+    std::shared_ptr<ShadowNode const> shadowNode);
+
 } // namespace react
 } // namespace facebook

ReactCommon/react/renderer/core/ShadowNodeTraits.h

@@ -21,7 +21,7 @@ class ShadowNodeTraits {
  public:
   /*
    * Underlying type for the traits.
-   * The first 18 bits are reserved for Core.
+   * The first 23 bits are reserved for Core.
    */
   enum Trait : int32_t {
     None = 0,
@@ -88,6 +88,16 @@ class ShadowNodeTraits {
 
     // Inherits 'TextShadowNode'
     Text = 1 << 17,
+
+    // Reserved
+    ReservedTrait1 = 1 << 18,
+    ReservedTrait2 = 1 << 19,
+    ReservedTrait3 = 1 << 20,
+    ReservedTrait4 = 1 << 21,
+    ReservedTrait5 = 1 << 22,
+
+    // Unserved - alias these for local usage
+    UnreservedTrait1 = 1 << 23
   };
 
   /*