[clang] [clang][dataflow] Model assignment to derived class from base. (PR #85064)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

Thanks! At this point all of my comments are addressed, and I am happy with the patch. 

https://github.com/llvm/llvm-project/pull/85064