because it was actually used in
https://github.com/tmm1/rbtrace/blob/v0.4.8/ext/rbtrace.c#L329
and deprecated in r60579 AFTER removal in r60558.

ko1 agreed that we should keep just deprecated in Ruby 2.5 and remove it
later, and I'm commiting this because I want to make rbtrace.gem
installation successful.

backward.h: modify r60579 to make rb_frame_method_id_and_class()
compilable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60964 b2dd03c8-39d4-4d8f-98ff-823fe69b080e