[Bug c++/65801] [5/6 Regression] Allow -Wno-narrowing to silence stricter C++11 narrowing rules

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65801

--- Comment #12 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Paolo Carlini from comment #11)
> Draft patch 2

I think let's go with this.  It's odd, but not complex and does what we want.