[libiberty] Improve copyright/license notice for cp-demangle.c

[DJ Delorie]

> DJ, is this OK?
> 
> -   This file is part of GNU CC.
> +   This file is part of the libiberty library, which is part of GCC.

I suspect this should say something like ", which is part of GCC, and
used in GDB, Binutils, and many other projects."  I don't know what
the exact list is, but it includes such projects as cygwin, sid, etc.

But your text is better than what is there, so go ahead and apply it.
We can always tweak it later if needed.