[Docs] Update clang & llvm release notes for HLSL

[beanz]

Summary

Adding release note entries for LLVM & Clang to introduce the HLSL &
DirectX support that is being added.

[aaron.ballman]

Thanks for adding the release notes! LGTM aside from some minor nits (take em or leave em).

clang/docs/ReleaseNotes.rst

72

75

[MaskRay]

clang/docs/ReleaseNotes.rst

72

The sentences may be simplified as:

Experimental support for HLSL has been added. <if needed, add more information how experimental it is>

The doc is about Clang, so many occurrences of Clang are redundant.

llvm/docs/ReleaseNotes.rst

119

DirectX has been added as an experimental target.
Specify `-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=DirectX` in your CMake configuration to enable it. The target is not packaged in pre-built binaries.

"LLVM tree" does not convey more information.

[beanz]

Updates based on review feedback. Thank you!

[MaskRay]

llvm/docs/ReleaseNotes.rst

120

Fix indentation and use two backsticks.

[beanz]

Updates based on feeback from @MaskRay. Thank you!

clang/docs/ReleaseNotes.rst

- Clang now supports randomizing structure layout in C. This feature is a
   on a seed value. Use the ``-frandomize-layout-seed=`` or
   ``-frandomize-layout-seed-file=`` flags.
 
   .. note::
 
       Randomizing structure layout is a C-only feature.
 
 - Clang now supports the ``-fstrict-flex-arrays=<arg>`` option to control which
   array bounds lead to flexible array members. The option yields more accurate
   ``__builtin_object_size`` and ``__builtin_dynamic_object_size`` results in
   most cases but may be overly conservative for some legacy code.
+- Experimental support for HLSL has been added. The implementation is
+  incomplete and highly experimental. For more information about the ongoing
+  work to support HLSL see the `documentation
+  <https://clang.llvm.org/docs/HLSLSupport.html>`_, or the `GitHub project
+  <https://github.com/orgs/llvm/projects/4>`_.
 
 Bug Fixes
 ---------
 - ``CXXNewExpr::getArraySize()`` previously returned a ``llvm::Optional``
   wrapping a ``nullptr`` when the ``CXXNewExpr`` did not have an array
   size expression. This was fixed and ``::getArraySize()`` will now always
   either return ``None`` or a ``llvm::Optional`` wrapping a valid ``Expr*``.
   This fixes `Issue 53742 <https://github.com/llvm/llvm-project/issues/53742>`_.

llvm/docs/ReleaseNotes.rst

 * Implemented generation of Windows SEH unwind information.
 * Switched the MinGW target to use SEH instead of DWARF for unwind information.
 
 Changes to the AVR Backend
 --------------------------
 
 * ...
 
+Changes to the DirectX Backend
+------------------------------
+
+* DirectX has been added as an experimental target. Specify
+  ``-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=DirectX`` in your CMake configuration
+  to enable it. The target is not packaged in pre-built binaries.
+* The DirectX backend supports the ``dxil`` architecture which is based on LLVM
+  3.6 IR encoded as bitcode and is the format used for DirectX GPU Shader
+  programs.
+
 Changes to the Hexagon Backend
 ------------------------------
 
 * ...
 
 Changes to the MIPS Backend
 ---------------------------