Add a couple of release note updates for C++ changes since Clang 4.

C++ Language Changes in Clang

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- Support for the C++17 standard has been completed. This mode can be enabled

  using ``-std=c++17`` (the old flag ``-std=c++1z`` is still supported for

  compatibility).

- When targeting a platform that uses the Itanium C++ ABI, Clang implements a

  `recent change to the ABI`__ that passes objects of class type indirectly if they

  have a non-trivial move constructor. Previous versions of Clang only

  considered the copy constructor, resulting in an ABI change in rare cases,

  but GCC has already implemented this change for several releases.

  This affects all targets other than Windows and PS4. You can opt out of this

  ABI change with ``-fclang-abi-compat=4.0``.

- As mentioned in `C Language Changes in Clang`_, Clang's support for

  implicit scalar to vector conversions also applies to C++. Additionally

  the following operators are also supported: ``&&`` and ``||``.

.. __: https://github.com/itanium-cxx-abi/cxx-abi/commit/7099637aba11fed6bdad7ee65bf4fd3f97fbf076

Objective-C Language Changes in Clang

-------------------------------------