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

[Baranov Victor via cfe-commits]

vbvictor wrote:

> I would prefer keeping this bugfix and that internal architectural improvement in two separate commits if that isn't too much trouble.

Yes, making `Signature` class should be a separate refactor change. I think we should come to a consensus in which checks (or a subset of functions inside checks) we want to have strict type-matching

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