[clang][dataflow]Use cast_or_null instead of cast to prevent crash (#68510)

`getStorageLocation` may return `nullptr` and this will produce crash
when use `cast`, use `dyn_cast_or_null` instead. I test it locally using
[FTXUI](https://github.com/ArthurSonzogni/FTXUI) and it may be the cause
of issue [issue](https://github.com/llvm/llvm-project/issues/68412), but
I am not sure.

Co-authored-by: huqizhi <huqizhi@836744285@qq.com>