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

[mstorsjo]

Summary

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

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.copy_file/copy_file_large.pass.cpp

if (space(env.test_root).available < 3 * test_file_size) {
     TEST_UNSUPPORTED();
   }
 
   // Create a file right at the size limit. The file is full of '\0's.
   const path file = env.create_file("source", sendfile_size_limit);
   const std::string additional_data(additional_size, 'x');
   // Append known data to the end of the source file.
   {
-    std::ofstream outf(file.native(), std::ios_base::app);
+    std::ofstream outf(file.string(), std::ios_base::app);
     TEST_REQUIRE(outf.good());
     outf << additional_data;
     TEST_REQUIRE(outf);
   }
   TEST_REQUIRE(file_size(file) == test_file_size);
   const path dest = env.make_env_path("dest");
 
   std::error_code ec = GetTestEC();
   TEST_CHECK(copy_file(file, dest, ec));
   TEST_CHECK(!ec);
 
   TEST_REQUIRE(is_regular_file(dest));
   TEST_CHECK(file_size(dest) == test_file_size);
 
   // Read the data from the end of the destination file, and ensure it matches
   // the data at the end of the source file.
   std::string out_data;
   out_data.reserve(additional_size);
   {
-    std::ifstream dest_file(dest.native());
+    std::ifstream dest_file(dest.string());
     TEST_REQUIRE(dest_file);
     dest_file.seekg(sendfile_size_limit);
     TEST_REQUIRE(dest_file);
     dest_file >> out_data;
     TEST_CHECK(dest_file.eof());
   }
   TEST_CHECK(out_data.size() == additional_data.size());
   TEST_CHECK(out_data == additional_data);
 }
 
 TEST_SUITE_END()