[clang] [clang][dataflow] Fix a new crash on assigning values of unmodeled types. (PR #179788)

[Artem Dergachev via cfe-commits]

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haoNoQ wrote:

We should probably do this too in the null case. But it's an urgent crash fix so let's make sure the crash is gone first.

https://github.com/llvm/llvm-project/pull/179788