libstdc++'s std::filesystem::path implementation incorrectly assumes
that any 8-bit char input is UTF-8, when it patently isn't on Windows.
Reported at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111244
Pick-to: 6.5 6.6
Fixes: QTBUG-116609
Change-Id: I2b24e1d3cad44897906efffd17803f2862935c9b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>