Add set_new_record to singular create associations

Fixes an issue where the foreign key wasn't being set, causing validation
to fail.  Eg:

```ruby

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post

  validates_presence_of :post_id

  def create_with_post
    new_post = create_post
    save
  end
end

class BugTest < Minitest::Test
  def test_association_stuff
    comment = Comment.new
    comment.create_with_post

    assert_predicate comment.post, :persisted?
    assert_equal Post.last, comment.post

    assert_predicate comment, :valid?
      # Failure:
      # BugTest#test_association_stuff [repro_fk_validation_change.rb:53]:
      # Expected #<Comment id: nil, post_id: nil> to be valid?.
    assert_predicate comment, :persisted?
  end
end
```

Author: nickh

activerecord/lib/active_record/associations/singular_association.rb

@@ -57,6 +57,7 @@ def _create_record(attributes, raise_error = false, &block)
           reflection.klass.transaction do
             record = build(attributes, &block)
             saved = record.save
+            set_new_record(record)
             raise RecordInvalid.new(record) if !saved && raise_error
             record
           end

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -1443,6 +1443,15 @@ def test_should_set_foreign_key_on_create_association!
     assert_equal firm.id, client.client_of
   end
 
+  def test_should_set_foreign_key_on_create_association_with_unpersisted_owner
+    tagging = Tagging.new
+    tag = tagging.create_tag
+
+    assert_not_predicate tagging, :persisted?
+    assert_predicate tag, :persisted?
+    assert_equal tag.id, tagging.tag_id
+  end
+
   def test_should_set_foreign_key_on_save
     client = Client.create! name: "fuu"
     firm   = client.build_firm name: "baa"