[Bug libstdc++/59042] Declaration of back_insert_iterator::value_type is incorrect

[cdodd at acm dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59042

--- Comment #1 from Chris Dodd <cdodd at acm dot org> ---
The other type traits of back_insert_iterator should probably not be void, as
well.