Move Arel::Nodes::LeadingJoin into arel.

- this way it is properly hidden from API documentation

Author: simi

activerecord/lib/active_record/relation/query_methods.rb

@@ -1517,9 +1517,6 @@ def select_association_list(associations, stashed_joins = nil)
         result
       end
 
-      class ::Arel::Nodes::LeadingJoin < Arel::Nodes::InnerJoin # :nodoc:
-      end
-
       def build_join_buckets
         buckets = Hash.new { |h, k| h[k] = [] }
 

activerecord/lib/arel/nodes.rb

@@ -63,6 +63,7 @@
 require "arel/nodes/outer_join"
 require "arel/nodes/right_outer_join"
 require "arel/nodes/string_join"
+require "arel/nodes/leading_join"
 
 require "arel/nodes/comment"
 

activerecord/lib/arel/nodes/leading_join.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+module Arel # :nodoc: all
+  module Nodes
+    class LeadingJoin < Arel::Nodes::InnerJoin
+    end
+  end
+end