[Bug libstdc++/83279] std::experimental::filesystem::copy_file can't copy larger files than 2.0GiB

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83279

--- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I forgot about that. I'll need to put the offset value back and then add a seek
to the input streambuf.