[Bug preprocessor/61389] libcpp diagnostics shouldn't talk about ISO C99 for C++ input files

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61389

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't have time to work on this, so feel free to take it - thanks!