[libc++] Implement file_clock::{to,from}_sys
This is part of https://wg21.link/P0355R7. I am adding these methods
to provide an alternative for the {from,to}_time_t methods that were
removed in https://llvm.org/D113027.
Differential Revision: https://reviews.llvm.org/D113430
NOKEYCHECK=True
GitOrigin-RevId: dce5fc56b6191ec63fd1d2bed7d3c815bacc49b7
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 9ccb211..62e5a0c 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -80,7 +80,9 @@
- Removed the nonstandard methods ``std::chrono::file_clock::to_time_t`` and
``std::chrono::file_clock::from_time_t``; neither libstdc++ nor MSVC STL
- had such methods.
+ had such methods. Instead, in C++20, you can use ``std::chrono::file_clock::from_sys``
+ and ``std::chrono::file_clock::to_sys``, which are specified in the Standard.
+ If you are not using C++20, you should move to it.
- The declarations of functions ``declare_reachable``, ``undeclare_reachable``, ``declare_no_pointers``,
``undeclare_no_pointers``, and ``get_pointer_safety`` have been removed not only from C++2b but