Update the existing modeling of stream functions in StreamChecker
with 'errno' modeling. The not (yet) evaluated stream functions
are not updated.
Have you considered using llvm::Optional<int> for this?
I believe, getDecl() might return null, e.g. for calling a function pointer.
So these two lines correspond to the following clang_analyzer_eval().
In this case the function should be known (we know that fopen is the called function, if the name is known the Decl should be known too), I think it can not be null here.