[Bug libstdc++/79980] Possible bug in codecvt.cpp bitmask setting code

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79980

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's just a dumb error where I used a typedef that was previously defined, the
fix is:

  static underlying_type<codecvt_mode>::type
  to_integer(codecvt_mode m)
  { return static_cast<underlying_type<codecvt_mode>::type>(m); }