Update ReleaseNotes.rst

Author: abhishek-kaushik22

clang/docs/ReleaseNotes.rst

@@ -229,6 +229,9 @@ Bug Fixes in This Version
   cast chain. (#GH149967).
 - Fixed a crash with incompatible pointer to integer conversions in designated
   initializers involving string literals. (#GH154046)
+- Clang now emits a frontend error when a function marked with the `flatten` attribute
+  calls another function that requires target features not enabled in the caller. This
+  prevents a fatal error in the backend.
 
 Bug Fixes to Compiler Builtins
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