[C++ Patch] Remove most uses of in_system_header_at

[Paolo Carlini]

.. hi again.

We have an issue with this idea which I didn't notice earlier today: 
there are some libstdc++ *_neg testcases which rely on permerrors being 
emitted for code in library headers. For example 
20_util/ratio/cons/cons_overflow_neg.cc relies on a permerror for the 
"overflow in constant expression" in constexpr.c. Or, 
20_util/variant/visit_neg.cc relies on the permerror for "invalid 
conversion from .. to .." emitted by convert_like_real.

Something seems a little fishy here but I'm not sure which way we want 
to go: I don't think we want to audit that right here, right now all the 
permerrors in the front-end which could potentially be involved in this 
kind of issue. Even if we, say, promote to plain errors the above two, 
that seems brittle, we got many permerrors which in principle should be 
real errors.

Paolo.