Add release note

Will be ABI breaking for Clang 20

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -160,6 +160,9 @@ Bug Fixes in This Version
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- ``__is_layout_compatible`` no longer requires the empty bases to be the same in two
+  standard-layout classes. It now only compares non-static data members.
+
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^