Fabric: *Informal* DebugStringConvertible interface

Summary:
Informal `DebugStringConvertible` interface serves the same purpose as `DebugStringConvertible` abstract class (providing universal pretty-printing interface) but relies on C++ static overloading instead of virtual dispatch.

This approach has some advantages and disadvantages:
Pros:
 * It's more clear and scoped. It's simpler to implement debug-printing functionality aside of normal production code.
* It's more composable.
* It allows print types that are not classes.
* For some classes using `DebugStringConvertible` means that we have to use virtual inheritance (which might be undesirable and affect *production* performance (if a compiler isn't smart enough to remove empty base class).
* For some highly lean classes (that designed to be as small as possible) adding base (even empty-in-prod) classes kinda... smells.

Cons:
The particular implementations cannot rely on dynamic dispatch which can complicate printing classes with meaningful differences between sampling classes (e.g. ShadowNode subclasses). That's why we don't remove `DebugStringConvertible` class yet.

Reviewed By: mdvacca

Differential Revision: D14715081

fbshipit-source-id: 1d397dbf81dc6d1dff0cc3f64ad09f10afe0085d

ReactCommon/fabric/debug/DebugStringConvertible.cpp

@@ -78,6 +78,19 @@ std::string DebugStringConvertible::getDebugDescription(
     DebugStringConvertibleOptions options) const {
   auto nameString = getDebugName();
   auto valueString = getDebugValue();
+
+  // Convention:
+  // If `name` and `value` are empty, `description` is also empty.
+  if (nameString.empty() && valueString.empty()) {
+    return "";
+  }
+
+  // Convention:
+  // If `name` is empty and `value` isn't empty, `description` equals `value`.
+  if (nameString.empty()) {
+    return valueString;
+  }
+
   auto childrenString = getDebugChildrenDescription(options);
   auto propsString = getDebugPropsDescription(options);