[Bug libstdc++/86015] Better handling of iterator distances

["joshua.r.marshall.1991 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86015

--- Comment #9 from Josh Marshall <joshua.r.marshall.1991 at gmail dot com> ---
Confirmed in 8.1.1

Also, looking over the following documents:
https://en.cppreference.com/w/cpp/types/ptrdiff_t
https://en.cppreference.com/w/cpp/iterator/iterator
http://en.cppreference.com/w/cpp/iterator/advance
http://en.cppreference.com/w/cpp/iterator/distance
https://en.cppreference.com/w/cpp/concept/Iterator
https://en.cppreference.com/w/cpp/iterator/iterator_traits

All of these do not require difference_type (typename _Distance in glibc) to be
convertible to an integer, and imply implicit conversion from an integer.  From
this reading, I need to change this from an enhancement to a bug.  I also need
to put in a request for a clarification to the standard on this, because it
wasn't very apparent.

Unfortunately, this involves the bigger change to glibc.  Functions like __lg()
will have to be re-written.