release note

Author: tbaederr

clang/docs/ReleaseNotes.rst

@@ -288,6 +288,10 @@ Improvements to Clang's diagnostics
 - Fixed a bug where the source location was missing when diagnosing ill-formed
   placeholder constraints.
 
+- The two-element, unary mask variant of ``__builtin_shufflevector`` is now
+  properly being rejected when used at compile-time. It was not implemented
+  and caused assertion failures before (#GH158471).
+
 Improvements to Clang's time-trace
 ----------------------------------