correctly mix in lazy val with overloaded def in subclass

2.12.0-RC1 incorrectly did not synthesize an implementation for the `lazy val overloaded`, because it only looked at result types to distinguish overloaded and overriding members... 😱 

```
trait T { lazy val overloaded: String = "a" }
class C extends T { def overloaded(a: String): String = "b" }
```
