Give GeneratedAttributeMethods module a name

Currently GeneratedAttributeMethods is a module builder class, an
instance of which is included in every AR class. OTOH,
GeneratedAssociatedMethods is assigned to a constant under the model
namespace. This is inconsistent and looks strange in the list of
ancestors.

There is no particular reason *not* to assign a constant for this (very
important) module under the model namespace, so that's what this commit
does.

Previous to this change, ancestors for an AR class looked like this:

```
=> [User (call 'User.connection' to establish a connection),
 User::GeneratedAssociationMethods,
 #<ActiveRecord::AttributeMethods::GeneratedAttributeMethods:0x000055ace0f05b08>,
 ApplicationRecord(abstract),
 ApplicationRecord::GeneratedAssociationMethods,
 #<ActiveRecord::AttributeMethods::GeneratedAttributeMethods:0x000055ace093c460>,
 ActiveRecord::Base,
 ...
```

With this change, they look like this:

```
=> [User (call 'User.connection' to establish a connection),
 User::GeneratedAssociationMethods,
 User::GeneratedAttributeMethods,
 ApplicationRecord(abstract),
 ApplicationRecord::GeneratedAssociationMethods,
 ApplicationRecord::GeneratedAttributeMethods,
 ActiveRecord::Base,
 ...
```

The previously named `GeneratedAttributeMethods` module builder class is
renamed `GeneratedAttributeMethodsBuilder` to emphasize that this is not
a module but a class.

activerecord/lib/active_record/attribute_methods.rb

@@ -24,7 +24,7 @@ module ActiveRecord
 
     RESTRICTED_CLASS_METHODS = %w(private public protected allocate new name parent superclass)
 
-    class GeneratedAttributeMethods < Module #:nodoc:
+    class GeneratedAttributeMethodsBuilder < Module #:nodoc:
       include Mutex_m
     end
 
@@ -35,7 +35,8 @@ module ActiveRecord
       end
 
       def initialize_generated_modules # :nodoc:
-        @generated_attribute_methods = GeneratedAttributeMethods.new
+        @generated_attribute_methods = const_set(:GeneratedAttributeMethods, GeneratedAttributeMethodsBuilder.new)
+        private_constant :GeneratedAttributeMethods
         @attribute_methods_generated = false
         include @generated_attribute_methods
 
@@ -88,7 +89,7 @@ module ActiveRecord
           # If ThisClass < ... < SomeSuperClass < ... < Base and SomeSuperClass
           # defines its own attribute method, then we don't want to overwrite that.
           defined = method_defined_within?(method_name, superclass, Base) &&
-            ! superclass.instance_method(method_name).owner.is_a?(GeneratedAttributeMethods)
+            ! superclass.instance_method(method_name).owner.is_a?(GeneratedAttributeMethodsBuilder)
           defined || super
         end
       end

activerecord/test/cases/attribute_methods_test.rb

@@ -1083,7 +1083,7 @@ class AttributeMethodsTest < ActiveRecord::TestCase
 
   test "generated attribute methods ancestors have correct class" do
     mod = Topic.send(:generated_attribute_methods)
-    assert_match %r(GeneratedAttributeMethods), mod.inspect
+    assert_match %r(Topic::GeneratedAttributeMethods), mod.inspect
   end
 
   private