Small hashset population improvement

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/Upgrade/Internals/HintGenerator.cs

@@ -357,7 +357,7 @@ private void GenerateCleanupByForeignKeyHints(StoredTypeInfo removedType, Cleanu
         ? removedType.AllDescendants
         : Array.Empty<StoredTypeInfo>();
       var descendants = new HashSet<StoredTypeInfo>(descendantsToHash.Length);
-      descendantsToHash.ForEach(t => descendants.Add(t));
+      descendants.UnionWith(descendantsToHash);
 
       var affectedAssociations = (
         from association in extractedModel.Associations