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

[Qizhi Hu via cfe-commits]

jcsxky wrote:

> The fix looks great to me! On the other hand, we usually try to add a regression test for each of the fixes. Any chance you could get a minimal reproducer from the codebase you are looking at?

Thanks for your suggestion! Test case has been added to reproduce this issue.

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