[PATCH] improve validation of attribute arguments (PR c/78666)

[Aldy Hernandez]

> Attached is another revision of this patch with the new helper
> function you suggested.  Is this version okay to commit?

I don't actually have the ability to grant check-in in this area.  I 
just figured that implementing some of these suggestions might make it 
more palatable to reviewers who do.

The patch is much better, but IMO it is a bit hard to read and not 
generic enough, but I defer to the relevant maintainers.

Aldy