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

[Alejandro Álvarez Ayllón via cfe-commits]

alejandro-alvarez-sonarsource wrote:

Sorry for the force-push, but I have moved now the precondition checks to `UnixAPIChecker` after uplifting its API (now it uses `PreCall` instead of `PreStmt`). From my understanding, the previous implementation used a legacy way of handling them.

The idea would be to rebase, keep the `[NFC]` separate, and squash everything that comes after.
Since I had a merge from main in the middle, I found it easier to rebase.

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