<chrono>: system_clock::now() and file_clock::from_utc() interpret FILETIME in different ways

[statementreply]

Describe the bug
In old Windows versions, FILETIME didn't count leap seconds. Since Windows 10 1809, Windows has become aware of leap seconds, and FILETIME counts leap seconds after 2018-06 by default (could be turned off system-wide to restore the old behavior).

Currently, system_clock::now() assumes the old behavior that FILETIME doesn't count leap seconds (#1520):

STL/stl/inc/chrono

Lines 666 to 668 in 4ee3f91

	_NODISCARD static time_point now() noexcept { // get current time
	return time_point(duration(_Xtime_get_ticks()));
	}

But file_clock::from_utc() and file_clock::to_utc() assume that FILETIME always counts leap seconds after 2017:

STL/stl/inc/chrono

Lines 3324 to 3325 in 4ee3f91

	// Assumes that FILETIME counts leap seconds only after the first 27 (i.e., after 1 January 2017), even though
	// systems can opt out of this behavior.

We should fix all of them to return the correct result whether or not the system is leap second aware.

STL version
4ee3f91