[clang] [clang][analyzer] fix crash when modelling 'getline' function in checkers (PR #145229)

[Balazs Benics via cfe-commits]

https://github.com/steakhal requested changes to this pull request.

Looks pretty good. I think we can simplify some code by using `dyn_cast_or_null` at a couple of places. I also found a bug.
I think this is heading to the right direction.

https://github.com/llvm/llvm-project/pull/145229