[Clang] Update ReleaseNotes

Signed-off-by: yronglin <[email protected]>

Author: yronglin

clang/docs/ReleaseNotes.rst

@@ -78,6 +78,9 @@ C++ Language Changes
 - Allow single element access of GCC vector/ext_vector_type object to be 
   constant expression. Supports the `V.xyzw` syntax and other tidbits 
   as seen in OpenCL. Selecting multiple elements is left as a future work.
+- Implement `CWG1815 <https://wg21.link/CWG1815>`_. Support lifetime extension 
+  of temporary created by aggregate initialization using a default member
+  initializer.
 
 C++17 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^