Print type of singleton that is requested after destruction

Summary: As titled
Trying to troubleshoot shutdown problems in HHVM and I see:
  EventBase::runAfterDelay() callback threw St13runtime_error exception: Raw pointer to a singleton requested after its destruction.
Would be much easier to troubleshoot if I know what is it being requested.

Reviewed By: @chipturner

Differential Revision: D2274497

folly/Singleton-inl.h

@@ -64,7 +64,9 @@ T* SingletonHolder<T>::get() {
 
   if (instance_weak_.expired()) {
     throw std::runtime_error(
-      "Raw pointer to a singleton requested after its destruction.");
+        "Raw pointer to a singleton requested after its destruction."
+        " Singleton type is: " +
+        type_.name());
   }
 
   return instance_ptr_;