[clang-tools-extra] [clang][dataflow]Use dyn_cast_or_null instead cast to prevent crash (PR #68510)

[Qizhi Hu via cfe-commits]

jcsxky wrote:

> please update release note also

Although `bugprone-unchecked-optional-access` is used to test this issue, root cause is in `FlowSensitiveAnalysis`. So, I am confused about which file should be updated about release not. Look forward to your suggestion! Thank you.

https://github.com/llvm/llvm-project/pull/68510