Fabric: Unifying usage of autos

Summary:
I was watching a classic magnificent talk about modern C++ by Herb Sutter and I was totally sold on double down on using `auto` in our codebase. Surprisingly, 95% of the code base already follows Herb's guidence; I just changed the last 5% to make it consistent.
All those changes must work *exactly* like it was before.
The talk: https://youtu.be/xnqTKD8uD64?t=28m25s

Reviewed By: mdvacca

Differential Revision: D9753301

fbshipit-source-id: 9629aa485a5d6e51806cc96306c297284d4f90b8

ReactCommon/fabric/attributedstring/AttributedString.cpp

@@ -40,7 +40,7 @@ const std::vector<Fragment> &AttributedString::getFragments() const {
 }
 
 std::string AttributedString::getString() const {
-  std::string string;
+  auto string = std::string {};
   for (const auto &fragment : fragments_) {
     string += fragment.string;
   }
@@ -50,7 +50,7 @@ std::string AttributedString::getString() const {
 #pragma mark - DebugStringConvertible
 
 SharedDebugStringConvertibleList AttributedString::getDebugChildren() const {
-  SharedDebugStringConvertibleList list = {};
+  auto list = SharedDebugStringConvertibleList {};
 
   for (auto &&fragment : fragments_) {
     auto propsList = fragment.textAttributes.DebugStringConvertible::getDebugProps();