Add release note

Author: DavidSpickett

clang/docs/ReleaseNotes.rst

@@ -395,6 +395,8 @@ Bug Fixes in This Version
 - Fixed a crash when trying to transform a dependent address space type. Fixes #GH101685.
 - Fixed a crash when diagnosing format strings and encountering an empty
   delimited escape sequence (e.g., ``"\o{}"``). #GH102218
+- The warning emitted for an unsupported register variable type now points to
+  the type instead of the ``register`` keyword (#GH109776).
 
 Bug Fixes to Compiler Builtins
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