[Bug c/80529] Split "C++ style comments are not allowed in ISO C90" pedwarn into its own warning flag

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80529

--- Comment #8 from Eric Gallager <egallager at gcc dot gnu.org> ---
My previous arguments for having a flag for this have been in the positive
form, i.e., so that it can be enabled separately, but I'd also like to state it
in its negative form, i.e., so that one can do -std=c89 -pedantic
-Wno-c++-style-comment to get the rest of the C89 pedwarns without also getting
the warnings for C++ style comments.