Use native Class#subclasses if available

Followup: https://github.com/rails/rails/pull/43548
Ref: https://github.com/ruby/ruby/pull/5045

That feature was accepted by Matz and should make it to Ruby 3.1.

We use feature testing just in case it wouldn't and also to better
support alternative implementations.

activesupport/lib/active_support.rb

@@ -117,8 +117,6 @@ module ActiveSupport
   def self.utc_to_local_returns_utc_offset_times=(value)
     DateAndTime::Compatibility.utc_to_local_returns_utc_offset_times = value
   end
-
-  @has_native_class_descendants = Class.method_defined?(:descendants) # RUBY_VERSION >= "3.1"
 end
 
 autoload :I18n, "active_support/i18n"

activesupport/lib/active_support/core_ext/class/subclasses.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "active_support/ruby_features"
+
 class Class
   # Returns an array with all classes that are < than its receiver.
   #
@@ -18,7 +20,7 @@ class Class
     ObjectSpace.each_object(singleton_class).reject do |k|
       k.singleton_class? || k == self
     end
-  end unless ActiveSupport.instance_variable_get(:@has_native_class_descendants) # RUBY_VERSION >= "3.1"
+  end unless ActiveSupport::RubyFeatures::CLASS_DESCENDANTS
 
   # Returns an array with the direct children of +self+.
   #
@@ -29,5 +31,5 @@ class Class
   #   Foo.subclasses # => [Bar]
   def subclasses
     descendants.select { |descendant| descendant.superclass == self }
-  end
+  end unless ActiveSupport::RubyFeatures::CLASS_SUBCLASSES
 end

activesupport/lib/active_support/descendants_tracker.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "weakref"
+require "active_support/ruby_features"
 
 module ActiveSupport
   # This module provides an internal implementation to track descendants
@@ -16,7 +17,7 @@ module ActiveSupport
       end
     end
 
-    if ActiveSupport.instance_variable_get(:@has_native_class_descendants) # RUBY_VERSION >= "3.1"
+    if RubyFeatures::CLASS_DESCENDANTS
       class << self
         def subclasses(klass)
           klass.subclasses
@@ -35,8 +36,10 @@ module ActiveSupport
         end
       end
 
-      def subclasses
-        descendants.select { |descendant| descendant.superclass == self }
+      unless RubyFeatures::CLASS_SUBCLASSES
+        def subclasses
+          descendants.select { |descendant| descendant.superclass == self }
+        end
       end
 
       def direct_descendants

activesupport/lib/active_support/ruby_features.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+module ActiveSupport
+  module RubyFeatures # :nodoc:
+    CLASS_DESCENDANTS = Class.method_defined?(:descendants) # RUBY_VERSION >= "3.1"
+    CLASS_SUBCLASSES = Class.method_defined?(:subclasses) # RUBY_VERSION >= "3.1"
+  end
+end