[Bug c++/91770] warn on #includes inside extern C regions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91770

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. the C++ standard says it's undefined to do this for any standard header:

[using.headers] in the standard says "A translation unit shall include a header
only outside of any declaration or definition"