[ruby/delegate] Fix DelegateClass block "method redefined" warning

This commit prevents "method redefined" warnings when overriding methods
within a `DelegateClass` block, such as in the following example:

  ```ruby
  Base = Class.new do
    def foo
      "foo"
    end
  end

  Overridden = DelegateClass(Base) do
    def foo
      super + "!"
    end
  end
  ```

Fixes https://bugs.ruby-lang.org/issues/19047.

https://github.com/ruby/delegate/commit/214fae86de

lib/delegate.rb

@@ -412,10 +412,12 @@ def DelegateClass(superclass, &block)
     end
     protected_instance_methods.each do |method|
       define_method(method, Delegator.delegating_block(method))
+      alias_method(method, method)
       protected method
     end
     public_instance_methods.each do |method|
       define_method(method, Delegator.delegating_block(method))
+      alias_method(method, method)
     end
   end
   klass.define_singleton_method :public_instance_methods do |all=true|

test/test_delegate.rb

@@ -29,6 +29,18 @@ class TestDelegateClass < Test::Unit::TestCase
     assert_equal(1, klass.new([1]).foo)
   end
 
+  def test_delegate_class_block_with_override
+    warning = EnvUtil.verbose_warning do
+      klass = DelegateClass(Array) do
+        def first
+          super.inspect
+        end
+      end
+      assert_equal("1", klass.new([1]).first)
+    end
+    assert_empty(warning)
+  end
+
   def test_systemcallerror_eq
     e = SystemCallError.new(0)
     assert((SimpleDelegator.new(e) == e) == (e == SimpleDelegator.new(e)), "[ruby-dev:34808]")