Update libc++ release notes

This patch updates the libc++ release notes.
I have picked out several features/fixes/updates
that I think are worth mentioning.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/branches/release_90@370514 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ReleaseNotes.rst

@@ -25,3 +25,36 @@ For more information about libc++, please see the `Libc++ Web Site
 
 What's New in Libc++ 9.0.0?
 ===========================
+
+Fixes
+-----
+
+* Minor fixes to ``std::chrono`` operators.
+* libc++ now correctly handles Objective-C++ ARC qualifiers in ``std::is_pointer``.
+* ``std::span`` general updates and fixes.
+* Updates to the ``std::abs`` implementation.
+* ``std::to_chars`` now adds leading zeros.
+* Ensure ``std::tuple`` is trivially constructible.
+* ``std::aligned_union`` now works in C++03.
+* Output of nullptr to ``std::basic_ostream`` is formatted properly.
+
+Features
+--------
+
+* Implemented P0608: sane variant converting constructor.
+* Added ``ssize`` function.
+* Added ``front`` and ``back`` methods in ``std::span``.
+* ``std::is_unbounded_array`` and ``std::is_bounded_array`` added to type traits.
+* ``std::atomic`` now includes many new features and specialization including improved Freestanding support.
+* Added ``std::midpoint`` and ``std::lerp`` math functions.
+* Added the function ``std::is_constant_evaluated``.
+* Erase-like algorithms now return size type.
+* Added ``contains`` method to container types.
+* ``std::swap`` is now a constant expression.
+
+Updates
+-------
+
+* libc++ dropped support for GCC 4.9; we now support GCC 5.1 and above.
+* libc++ added explicit support for WebAssembly System Interface (WASI).
+* Progress towards full support of rvalues and variadics in C++03 mode. ``std::move`` and ``std::forward`` now both work in C++03 mode.