Add release note entry

Author: yuxuanchen1997

clang/docs/ReleaseNotes.rst

@@ -591,6 +591,8 @@ Bug Fixes in This Version
 
 - Fixed an issue that a benign assertion might hit when instantiating a pack expansion
   inside a lambda. (`#61460 <https://github.com/llvm/llvm-project/issues/61460>`_)
+- Fixed a crash during instantiation of nested class template specializations.
+  Fixes (`#70375 <https://github.com/llvm/llvm-project/issues/70375>`_)
 
 Bug Fixes to Compiler Builtins
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