[libcxx] [test] Use string() instead of native() as parameter to...

[libcxx] [test] Use string() instead of native() as parameter to std::ifstream/ofstream in copy_file_large.pass.cpp

This fixes building with libstdc++ for windows. MS STL has got
ifstream/ofstream overloads that taken wide strings though.

Differential Revision: https://reviews.llvm.org/D89539