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

[Balazs Benics via cfe-commits]

steakhal wrote:

Ideally, we could reuse the signature matching of the `StdLibraryFunctionsChecker`. It shouldn't be particularly difficult to hoist it, but also a considerable work by the size of the change.
I think the `Checkers` library component (or directory) is suitable for it. We could call it `Signature.{cpp,h}`.

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