alias_attribute: handle user defined source methods

`alias_attribute` used to define a "jump method", e.g.
`alias_attribute :foo, :bar` was pretty much a macro to generate

```ruby
def foo
  bar
end
```

This is convienient because it's easy, it doesn't impose an order
of declaration or anything like that.

But it's also much less efficient than a true `alias_method`.

It also used to cause cache size explosion which we fixed in
rails#52118, but making it behave
like Ruby's `alias_method`, by doing a real alias.

But this breaks some expectations (literally from the documentation):

```ruby
  attr_accessor :name
  attribute_method_suffix '_short?'
  define_attribute_methods :name

  alias_attribute :nickname, :name
```

Here we're not supposed to alias a generated method, but a user defined one.

So this assumption can only hold for Active Record, not Active Model.

activemodel/lib/active_model/attribute_methods.rb

@@ -215,7 +215,12 @@ def eagerly_generate_alias_attribute_methods(new_name, old_name) # :nodoc:
       end
 
       def generate_alias_attribute_methods(code_generator, new_name, old_name)
-        define_attribute_method(old_name, _owner: code_generator, as: new_name)
+        ActiveSupport::CodeGenerator.batch(code_generator, __FILE__, __LINE__) do |owner|
+          attribute_method_patterns.each do |pattern|
+            alias_attribute_method_definition(code_generator, pattern, new_name, old_name)
+          end
+          attribute_method_patterns_cache.clear
+        end
       end
 
       def alias_attribute_method_definition(code_generator, pattern, new_name, old_name) # :nodoc:
@@ -228,7 +233,7 @@ def alias_attribute_method_definition(code_generator, pattern, new_name, old_nam
         call_args = []
         call_args << parameters if parameters
 
-        define_call(code_generator, method_name, target_name, mangled_name, parameters, call_args, namespace: :alias_attribute)
+        define_call(code_generator, method_name, target_name, mangled_name, parameters, call_args, namespace: :alias_attribute, as: method_name)
       end
 
       # Is +new_name+ an alias?
@@ -441,7 +446,7 @@ def build_mangled_name(name)
           mangled_name = name
 
           unless NAME_COMPILABLE_REGEXP.match?(name)
-            mangled_name = "__temp__#{name.unpack1("h*")}"
+            mangled_name = :"__temp__#{name.unpack1("h*")}"
           end
 
           mangled_name

activemodel/test/cases/attribute_methods_test.rb

@@ -371,4 +371,44 @@ def attribute?(name)
     assert_equal :model_1, NameClash::Model1.new.x_changed?
     assert_equal :model_2, NameClash::Model2.new.x_changed?
   end
+
+  test "alias attribute respects user defined method" do
+    model = Class.new do
+      include ActiveModel::AttributeMethods
+
+      attr_accessor :name
+      define_attribute_methods :name
+
+      alias_attribute :nickname, :name
+
+      def initialize(name)
+        @name = name
+      end
+    end
+
+    instance = model.new("George")
+    assert_equal "George", instance.name
+    assert_equal "George", instance.nickname
+  end
+
+  test "alias attribute respects user defined method in parent classes" do
+    model = Class.new do
+      include ActiveModel::AttributeMethods
+
+      attr_accessor :name
+      define_attribute_methods :name
+
+      def initialize(name)
+        @name = name
+      end
+    end
+
+    subclass = Class.new(model) do
+      alias_attribute :nickname, :name
+    end
+
+    instance = subclass.new("George")
+    assert_equal "George", instance.name
+    assert_equal "George", instance.nickname
+  end
 end