[Bug libstdc++/67440] pretty-printing of a const set<foo> fails

[dje at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67440

--- Comment #1 from Doug Evans <dje at google dot com> ---
Created attachment 36288
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36288&action=edit
patch + testcase

This could use another set of eyes.