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

[3dw4rd at verizon dot net]

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

--- Comment #7 from Ed Smith-Rowland <3dw4rd at verizon dot net> ---
Add a dependency: 44317

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44317

At least it's closely related.