Fix singleton class cloning

	Before this commit, `clone` gave different results depending on whether the original object
	had an attached singleton class or not.

	Consider the following setup:
	```
	class Foo; end
	Foo.singleton_class.define_method(:foo) {}

	obj = Foo.new

	obj.singleton_class if $call_singleton

	clone = obj.clone
	```

	When `$call_singleton = false`, neither `obj.singleton_class.singleton_class` nor
	`clone.singleton_class.singleton_class` own any methods.

	However, when `$call_singleton = true`, `clone.singleton_class.singleton_class` would own a copy of
	`foo` from `Foo.singleton_class`, even though `obj.singleton_class.singleton_class` does not.

	The latter case is unexpected and results in a visibly different clone, depending on if the original object
	had an attached class or not.

Co-authored-by: Ufuk Kayserilioglu <ufuk.kayserilioglu@shopify.com>
	---
	 class.c                 | 31 ++++++++++++++++++++++---------
	 test/ruby/test_class.rb | 47 +++++++++++++++++++++++++++++++++++++++++++++++
	 2 files changed, 69 insertions(+), 9 deletions(-)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67935 b2dd03c8-39d4-4d8f-98ff-823fe69b080e