[Bug libstdc++/61347] std::distance(list.first(),list.end()) in O(1)

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61347

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
Created attachment 35231
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35231&action=edit
Overload of __distance

Here is one way to do it (I will probably protect the new code with
defined(__OPTIMIZE__) as well, if we keep __builtin_constant_p).

(Looking at the generated code, I was surprised to notice that _M_hook and
_M_unhook are in src, they seem rather small for that)