[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 #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I have a patch for replacing all the mentions of C99 with C++03 or C++11 but it
needs a little more work to be translation-friendly