[libcxx] [test] Mark tests for to_time_t as libcpp specific

[mstorsjo]

Summary

libstdc++ on linux doesn't provide it; I'm a bit unsure how to navigate all the overloads in the documentation to see whether it really is mandated or not.

[ldionne]

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

388

I think you're right, it's only specified as a member of system_clock. @EricWF is there a reason why you provided it to_time_t for your _FilesystemClock?

[ldionne]

This LGTM cause we're not supposed to provide to_time_t for the filesystem clock, but @mstorsjo would you mind trying to remove to_time_t from _FilesystemClock completely to see if that breaks anything serious?

[mstorsjo]

Superseded by D113027.

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

TEST_CASE(get_last_write_time_dynamic_env_test)
     const path dir = env.create_dir("dir");
 
     const auto file_times = GetTimes(file);
     const TimeSpec file_write_time = file_times.write;
     const auto dir_times = GetTimes(dir);
     const TimeSpec dir_write_time = dir_times.write;
 
     file_time_type ftime = last_write_time(file);
-    TEST_CHECK(Clock::to_time_t(ftime) == file_write_time.tv_sec);
+    LIBCPP_ONLY(TEST_CHECK(Clock::to_time_t(ftime) == file_write_time.tv_sec));
     TEST_CHECK(CompareTime(ftime, file_write_time));
 
     file_time_type dtime = last_write_time(dir);
-    TEST_CHECK(Clock::to_time_t(dtime) == dir_write_time.tv_sec);
+    LIBCPP_ONLY(TEST_CHECK(Clock::to_time_t(dtime) == dir_write_time.tv_sec));
     TEST_CHECK(CompareTime(dtime, dir_write_time));
 
     SleepFor(Sec(2));
 
     // update file and add a file to the directory. Make sure the times increase.
     std::FILE* of = std::fopen(file.string().c_str(), "a");
     std::fwrite("hello", 1, sizeof("hello"), of);
     std::fclose(of);