Add entry to release notes

Author: pranavk

clang/docs/ReleaseNotes.rst

@@ -253,6 +253,7 @@ ABI Changes in This Version
 - Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.
 - Fixed the Itanium mangling of the construction vtable name. This change will introduce incompatibilities with code compiled by Clang 19 and earlier versions, unless the -fclang-abi-compat=19 option is used. (#GH108015)
 - Mangle member-like friend function templates as members of the enclosing class. (#GH110247, #GH110503)
+- Return larger CXX records in memory instead of using AVX registers. Code compiled with older clang will be incompatible with newer version of the clang unless -fclang-abi-compat=19 is provided. (#GH120670)
 
 AST Dumping Potentially Breaking Changes
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