Fix type inferencing for method generics determined from functional elements

[eric-milles]

Consider the following:

class Named { String name }
@groovy.transform.TypeChecked
void test() {
  def c = Collectors.groupingBy(Named::getName)
}

The type checker is able to work out the collector type arguments as evidenced by hovering over the variable.

However the type given for the method is not quite complete.

[eric-milles]