PATCH: Support C++ in demanle.h

[DJ Delorie]

> I have no problems with either way. I just want to be
> consistent. Every file under include/ has the same style. I don't
> see why demangle.h should be any different.

Less than half the headers have the C++ guards at all, and some that
do have the includes (or at least some of the includes) outside the
guards.  Feel free to propose a patch to clean them up.

FYI the reason we shouldn't guard includes - in general - is because
the included files might need to prototype C++ functions.  It doesn't
happen to be the case in libiberty, but it's a bad habit to get into.

Meanwhile, if you move the guard past the includes in demangle.h, you
can commit that patch.

> BTW, I'd like to fix 3.3 too.

Likewise for demangle.h.  I see no pressing need to clean up the
others here.