Update libc++ release notes

[zoecarver]

Summary

This patch simply updates the release notes for libc++ 9.0. I have
picked out a few featues/fixes/updates that I think are worth
mentioning. Feel free to suggest changes, or simply leave a comment with
something that should be added and I will update this patch.

[thakis]

libcxx/docs/ReleaseNotes.rst

45

afaik `foo` is a link in rst and ``foo`` is code

[hans]

Thanks for writing release notes! Please let me know if you need assistance with committing to the branch.

libcxx/docs/ReleaseNotes.rst

45

I'm not sure if it's a link, I think it ends up using italics or something.

But +1, please use `foo` instead.

[jfb]

libcxx/docs/ReleaseNotes.rst

58

This seems misleading if it refers to my llvm version bump? The runtime is compiled with llvm so it’s true there, but users of older GCC should still be able to use the libc++ headers I think.

[zoecarver]

libcxx/docs/ReleaseNotes.rst

58

I was referring to a commit Eric made. Users of older GCC versions _will not_ be able to use libc++ headers anymore (because of constexpr, variadics, rvalues, etc.).

[zoecarver]

• update back-tick links to be code

[zoecarver]

Committed as rL370514.

libcxx/docs/ReleaseNotes.rst

 be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.
 
 For more information about libc++, please see the `Libc++ Web Site
 <https://libcxx.llvm.org>`_ or the `LLVM Web Site <https://llvm.org>`_.
 
 
 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.
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