[Bug c++/60304] Including <atomic> disables -Wconversion-null

["manu at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60304

--- Comment #9 from Manuel LÃpez-IbÃÃez <manu at gcc dot gnu.org> ---
(In reply to Paolo Carlini from comment #5)
> Well, of course the user can always explicitly include, eg, <cstdbool>, thus
> it seems that the real underlying issue is that the system-headers machinery
> should not be fooled by things like this in a system header... or should it?
> The define is rather interesting...
> 
> #define false false
> 
> I'm adding in CC Dodji too...

In the case of NULL we do:

      source_location loc =
    expansion_point_location_if_in_system_header (input_location);

however, we do not do it in the case of 'false' (because we do not think it
should be a macro, but it actually is). Perhaps we should do it, is there a
downside to it?