[Bug c++/89923] printf format check and char8_t

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89923

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Especially as the C++2a change breaks previously valid code:

printf("%s\n", u8"");