load.c: backtrace of circular require

	* load.c (load_lock): print backtrace of circular require via
	  `Warning.warn` [ruby-core:80850] [Bug #13505]

	Send the backtrace of the circular require warning as a single String to Warning.warn

	* load.c: send as a single string.
	* error.c: expose the string formatted by rb_warning as rb_warning_string().
	* test/ruby/test_exception.rb: update tests.
	  [ruby-core:80850] [Bug #13505]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62435 b2dd03c8-39d4-4d8f-98ff-823fe69b080e