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

[egall at gwmail dot gwu.edu]

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

--- Comment #2 from Eric Gallager <egall at gwmail dot gwu.edu> ---
(In reply to Jonathan Wakely from comment #1)
> IMHO warnings should not be enforcing in-house coding guidelines. Use
> clang-tidy for that.

GCC already has several warnings added for that purpose. -Wtemplates,
-Wmultiple-inheritance, -Wvirtual-inheritance, and -Wnamespaces are all
documented as existing because:
"Some coding rules disallow
[templates|[multiple|virtual]inheritance|namespaces], and this may be used to
enforce that rule."
And then -Weffc++ is also documented as being a set of style guidelines.
Also it seems odd that a GCC developer would recommend using a clang-based tool
instead of GCC when clang is a competing project designed specifically to
undermine GCC.