False negative in or_fn_call pertaining to nested ctors

[Manishearth]

#4018 fixes the main false positive in or_fn_call when it comes to things like .or(Some(foo)), keeping them exempt from the lint. However, in the process it also ignores .or(Some(foo.clone()), which it shouldn't.

We should have a visitor there that ensures that all expressions are either paths or ctors.