Assertion failure because we don't handle polymorphic parameters in collect_unknown_apply_args

[lpw25]

Whilst reviewing the modular explicits PR I noticed that it was possible to cause an assertion failure that I previously thought could not be reached. The following code causes the issue:

let foo f a =
  let _ = (f ~a : ('a. 'a -> 'a) -> int) in
  f ~a (fun x -> x)

This code causes us to reach the collect_unkown_apply_args code with an expected type that has a polymorphic parameter. We should extend the code to support such cases. The new cases should always consider the type information to be non-principal.