Fix TopLevelVisitor adding existing ClassDef type to current scope

spec/compiler/semantic/alias_spec.cr

@@ -178,6 +178,22 @@ describe "Semantic: alias" do
         Bar.bar
         )) { int32 }
     end
+
+    it "reopens #{type} through alias within itself" do
+      assert_type <<-CRYSTAL { int32 }
+        #{type} Foo
+          alias Bar = Foo
+
+          #{type} Bar
+            def self.bar
+              1
+            end
+          end
+        end
+
+        Foo.bar
+        CRYSTAL
+    end
   end
 
   %w(class struct).each do |type|

src/compiler/crystal/semantic/top_level_visitor.cr

@@ -193,9 +193,9 @@ class Crystal::TopLevelVisitor < Crystal::SemanticVisitor
       if superclass.is_a?(GenericClassInstanceType)
         superclass.generic_type.add_subclass(type)
       end
+      scope.types[name] = type
     end
 
-    scope.types[name] = type
     node.resolved_type = type
 
     process_annotations(annotations) do |annotation_type, ann|