Add a TODO comment to `FrozenStringLiteral` module
Add a TODO comment to `FrozenStringLiteral` module

Follow up of #5036.

This PR adds a memorandum to source code.

If frozen string literal is default in Ruby 3.0 it will remove
this TODO comment and if not default it will need to change
the conditional expression of `return true if ruby_version >= 3.0`.

petehamilton/rubocop,deivid-rodriguez/rubocop,jfelchner/rubocop,deivid-rodriguez/rubocop,rrosenblum/rubocop,akihiro17/rubocop,palkan/rubocop,palkan/rubocop,bbatsov/rubocop,tejasbubane/rubocop,bquorning/rubocop,bquorning/rubocop,mikegee/rubocop,sue445/rubocop,jmks/rubocop,panthomakos/rubocop,panthomakos/rubocop,jfelchner/rubocop,petehamilton/rubocop,deivid-rodriguez/rubocop,vergenzt/rubocop,tejasbubane/rubocop,rrosenblum/rubocop,panthomakos/rubocop,maxjacobson/rubocop,scottmatthewman/rubocop,pocke/rubocop,smakagon/rubocop,palkan/rubocop,mikegee/rubocop,smakagon/rubocop,tdeo/rubocop,bquorning/rubocop,mikegee/rubocop,tdeo/rubocop,tejasbubane/rubocop,maxjacobson/rubocop,jfelchner/rubocop,meganemura/rubocop,haziqhafizuddin/rubocop,meganemura/rubocop,vergenzt/rubocop,meganemura/rubocop,tdeo/rubocop,pocke/rubocop,vergenzt/rubocop,bbatsov/rubocop,akihiro17/rubocop,urbanautomaton/rubocop,jmks/rubocop,maxjacobson/rubocop,sue445/rubocop,petehamilton/rubocop,haziqhafizuddin/rubocop,rrosenblum/rubocop,urbanautomaton/rubocop,scottmatthewman/rubocop,jmks/rubocop,akihiro17/rubocop,sue445/rubocop