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.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271155 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f