[clang] [clang][analyzer] Model more getline/getdelim pre and postconditions (PR #83027)

[Alejandro Álvarez Ayllón via cfe-commits]

alejandro-alvarez-sonarsource wrote:

@balazske would you agree with my proposal of keeping this logic in `UnixAPIChecker`? I am also happy with adding more NULL checks to `StreamChecker`, but I can understand your concerns about overreaching its scope.

https://github.com/llvm/llvm-project/pull/83027