[clang] [clang][analyzer] Improve StdLibraryFunctionsChecker 'readlink' modeling. (PR #71373)

[Balazs Benics via cfe-commits]

https://github.com/steakhal approved this pull request.

LGTM, but please wait for @DonatNagyE to have a look.
BTW I noticed that the note messages are not tested, but I'm okay to not test that I think.

https://github.com/llvm/llvm-project/pull/71373