Schedule destroyInstances in registrationComplete

Summary: This makes it more likely that SingletonVaultDestructor won't be created after atexit calls.

Test Plan: unit test

Reviewed By: chip@fb.com

Subscribers: hphp-diffs@, ps, njormrod, folly-diffs@

FB internal diff: D1629804

Tasks: 5353022

folly/experimental/Singleton.h

@@ -208,6 +208,8 @@ class SingletonVault {
   // Mark registration is complete; no more singletons can be
   // registered at this point.
   void registrationComplete() {
+    scheduleDestroyInstances();
+
     RWSpinLock::WriteHolder wh(&stateMutex_);
 
     stateCheck(SingletonVaultState::Running);