Roll back r271012 even more aggressively.

I've looked at the GCC sources and I now understand what's going wrong.
THe C11 keywords are simply nonexistent when using C++ mode. They are
marked as C-only in the parser. This is absolutely impractical for
multiple reasons:

• The C11 keywords do not conflict with C++ naming rules. They all start with _[A-Z]. There is no reason to make them C-only.

• It makes it practically impossible for people to use these keywords in C header files and expect them to work from within C++ sources.

As I said in my previous commit message: GCC is by far the weirdest
compiler that I've ever used.