New demangler in C++

[Benjamin Kosnik]

>| The key point is that I think it's a big mistake to have two
>| demanglers.  If we're going to write a new one, we should make sure that
>| it can replace the old one, not just get added to the mix.
>
>This is OK fine by me.
>
>Benjamin?

I'm not talking about anything besides the __cxa_demangle entry point
as defined by the ABI. This function is already defined as extern "C".

I'm not going to write a new demangler. 

Instead, a new implementation was donated to the FSF that is
substantially better that what was used previously. It offers several
advantages, which I have already pointed out. I think it would be
foolish to ignore a gift and not use the new implementation for
__cxa_demangle.

This is now in.

best,
benjamin