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

[redi at gcc dot gnu.org]

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
IMHO warnings should not be enforcing in-house coding guidelines. Use
clang-tidy for that.