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

[Balázs Kéri via cfe-commits]

balazske wrote:

I want to avoid that some functions have null pointer checks in `StreamChecker`, some not. If this change is merged then it would be good to add null pointer checks to other functions like `fread` and `fwrite`. (Until now only the NULL stream pointer was checked.)

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