Suppress Layout/ClassStructure offenses

Follow up rubocop/rubocop#11329

```console
% bundle exec rake
(snip)

Offenses:

lib/rubocop/ast/node_pattern.rb:105:7: C: [Correctable] Layout/ClassStructure: public_class_methods is supposed to appear before initializer.
      def self.descend(element, &block) ...
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/rubocop/ast/processed_source.rb:23:7: C: [Correctable] Layout/ClassStructure: constants is supposed to appear before public_class_methods.
      INVALID_LEVELS = %i[error fatal].freeze
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tasks/changelog.rb:121:3: C: [Correctable] Layout/ClassStructure: public_class_methods is supposed to appear before initializer.
  def self.pending? ...
  ^^^^^^^^^^^^^^^^^

162 files inspected, 3 offenses detected, 3 offenses autocorrectable
```

lib/rubocop/ast/node_pattern.rb

@@ -54,6 +54,22 @@ def def_node_search(method_name, pattern_str, **keyword_defaults)
 
       VAR = 'node'
 
+      # Yields its argument and any descendants, depth-first.
+      #
+      def self.descend(element, &block)
+        return to_enum(__method__, element) unless block
+
+        yield element
+
+        if element.is_a?(::RuboCop::AST::Node)
+          element.children.each do |child|
+            descend(child, &block)
+          end
+        end
+
+        nil
+      end
+
       attr_reader :pattern, :ast, :match_code
 
       def_delegators :@compiler, :captures, :named_parameters, :positional_parameters
@@ -100,22 +116,6 @@ def init_with(coder) # :nodoc:
         initialize(coder['pattern'])
       end
 
-      # Yields its argument and any descendants, depth-first.
-      #
-      def self.descend(element, &block)
-        return to_enum(__method__, element) unless block
-
-        yield element
-
-        if element.is_a?(::RuboCop::AST::Node)
-          element.children.each do |child|
-            descend(child, &block)
-          end
-        end
-
-        nil
-      end
-
       def freeze
         @match_code.freeze
         @compiler.freeze

lib/rubocop/ast/processed_source.rb

@@ -12,6 +12,9 @@ class ProcessedSource
       # @api private
       STRING_SOURCE_NAME = '(string)'
 
+      INVALID_LEVELS = %i[error fatal].freeze
+      private_constant :INVALID_LEVELS
+
       attr_reader :path, :buffer, :ast, :comments, :tokens, :diagnostics,
                   :parser_error, :raw_source, :ruby_version
 
@@ -20,9 +23,6 @@ def self.from_file(path, ruby_version)
         new(file, ruby_version, path)
       end
 
-      INVALID_LEVELS = %i[error fatal].freeze
-      private_constant :INVALID_LEVELS
-
       def initialize(source, ruby_version, path = nil)
         # Defaults source encoding to UTF-8, regardless of the encoding it has
         # been read with, which could be non-utf8 depending on the default

tasks/changelog.rb

@@ -77,6 +77,21 @@ def github_user
       user
     end
   end
+
+  def self.pending?
+    entry_paths.any?
+  end
+
+  def self.entry_paths
+    Dir["#{ENTRIES_PATH}*"]
+  end
+
+  def self.read_entries
+    entry_paths.to_h do |path|
+      [path, File.read(path)]
+    end
+  end
+
   attr_reader :header, :rest
 
   def initialize(content: File.read(PATH), entries: Changelog.read_entries)
@@ -118,20 +133,6 @@ def merge_content
     ].join("\n")
   end
 
-  def self.pending?
-    entry_paths.any?
-  end
-
-  def self.entry_paths
-    Dir["#{ENTRIES_PATH}*"]
-  end
-
-  def self.read_entries
-    entry_paths.to_h do |path|
-      [path, File.read(path)]
-    end
-  end
-
   def new_contributor_lines
     contributors
       .map { |user| format(CONTRIBUTOR, user: user) }