Fix return value of 'PluginManager::RegisterPlugin()'. (#114120)

Author: Miro Bucko

lldb/source/Core/PluginManager.cpp

@@ -206,10 +206,9 @@ template <typename Instance> class PluginInstances {
     if (!callback)
       return false;
     assert(!name.empty());
-    Instance instance =
-        Instance(name, description, callback, std::forward<Args>(args)...);
-    m_instances.push_back(instance);
-    return false;
+    m_instances.emplace_back(name, description, callback,
+                             std::forward<Args>(args)...);
+    return true;
   }
 
   bool UnregisterPlugin(typename Instance::CallbackType callback) {