[sourcekit] Use a shared_ptr for the NotificationCenter

When the server shuts down we may still have outstanding async work that
can attempt to trigger a notification, so use a shared_ptr + weak_ptr
instead of unique_ptr + unowned references.

unittests/SourceKit/SwiftLang/CursorInfoTest.cpp

@@ -122,7 +122,7 @@ public:
   }
 
   void addNotificationReceiver(DocumentUpdateNotificationReceiver Receiver) {
-    Ctx.getNotificationCenter().addDocumentUpdateNotificationReceiver(Receiver);
+    Ctx.getNotificationCenter()->addDocumentUpdateNotificationReceiver(Receiver);
   }
 
   void open(const char *DocName, StringRef Text,