vm_args.c: search symbol proc in super classes

	* vm_args.c (refine_sym_proc_call): traverse ancestors to search
	  inherited methods for symbol proc.
	  [ruby-dev:50741] [Bug #15489]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66925 b2dd03c8-39d4-4d8f-98ff-823fe69b080e