Move column_for_attribute into ModelSchema

`type_for_attribute` was added (extracted from `column_for_attribute`)
in `ModelSchema` at c93dbfe, I sometimes had expected that both
`column_for_attribute` and `type_for_attribute` exists in `ModelSchema`,
but `column_for_attribute` had existed in `AttributeMethods` even though
it's not very relevant to attribute methods (getting column object is no
longer useful for most end users since type object is separated from
column object, by the way).

So I've moved `column_for_attribute` to the same place with
`type_for_attribute` to less confusion a bit.

activerecord/lib/active_record/attribute_methods.rb

@@ -19,8 +19,6 @@ module ActiveRecord
       include TimeZoneConversion
       include Dirty
       include Serialization
-
-      delegate :column_for_attribute, to: :class
     end
 
     RESTRICTED_CLASS_METHODS = %w(private public protected allocate new name parent superclass)
@@ -188,26 +186,6 @@ module ActiveRecord
       def _has_attribute?(attr_name) # :nodoc:
         attribute_types.key?(attr_name)
       end
-
-      # Returns the column object for the named attribute.
-      # Returns a +ActiveRecord::ConnectionAdapters::NullColumn+ if the
-      # named attribute does not exist.
-      #
-      #   class Person < ActiveRecord::Base
-      #   end
-      #
-      #   person = Person.new
-      #   person.column_for_attribute(:name) # the result depends on the ConnectionAdapter
-      #   # => #<ActiveRecord::ConnectionAdapters::Column:0x007ff4ab083980 @name="name", @sql_type="varchar(255)", @null=true, ...>
-      #
-      #   person.column_for_attribute(:nothing)
-      #   # => #<ActiveRecord::ConnectionAdapters::NullColumn:0xXXX @name=nil, @sql_type=nil, @cast_type=#<Type::Value>, ...>
-      def column_for_attribute(name)
-        name = name.to_s
-        columns_hash.fetch(name) do
-          ConnectionAdapters::NullColumn.new(name)
-        end
-      end
     end
 
     # A Person object with a name attribute can ask <tt>person.respond_to?(:name)</tt>,

activerecord/lib/active_record/model_schema.rb

@@ -141,7 +141,7 @@ module ActiveRecord
       self.inheritance_column = "type"
       self.ignored_columns = [].freeze
 
-      delegate :type_for_attribute, to: :class
+      delegate :type_for_attribute, :column_for_attribute, to: :class
 
       initialize_load_schema_monitor
     end
@@ -400,6 +400,26 @@ module ActiveRecord
         end
       end
 
+      # Returns the column object for the named attribute.
+      # Returns a +ActiveRecord::ConnectionAdapters::NullColumn+ if the
+      # named attribute does not exist.
+      #
+      #   class Person < ActiveRecord::Base
+      #   end
+      #
+      #   person = Person.new
+      #   person.column_for_attribute(:name) # the result depends on the ConnectionAdapter
+      #   # => #<ActiveRecord::ConnectionAdapters::Column:0x007ff4ab083980 @name="name", @sql_type="varchar(255)", @null=true, ...>
+      #
+      #   person.column_for_attribute(:nothing)
+      #   # => #<ActiveRecord::ConnectionAdapters::NullColumn:0xXXX @name=nil, @sql_type=nil, @cast_type=#<Type::Value>, ...>
+      def column_for_attribute(name)
+        name = name.to_s
+        columns_hash.fetch(name) do
+          ConnectionAdapters::NullColumn.new(name)
+        end
+      end
+
       # Returns a hash where the keys are column names and the values are
       # default values when instantiating the Active Record object for this table.
       def column_defaults