Hi,

in this issue Marc noticed that the warning for ignored attribute 
visibility is truncated when the previous declaration belongs to a 
system header. Luckily the problem can be neatly fixed because it's just 
a consequence of not using warning_at + inform, as we should anyway. The 
other hunk in the patch tweaks the only other remaining warning_at pair 
I found in the file, should be a rather benign one, but better adjusting 
it too.

Tested x86_64-linux.

Thanks,
Paolo.

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