[Bug libstdc++/79820] std::ifstream sets errno to zero

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79820

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I was wrong, and that code path is used by std::sync_with_stdio(false) when we
reset cin, cout, cerr and clog to use stdio_filebuf stream buffers. So there's
no need for a testcase.