Simplify simple forSome existential types into wildcard existentials

[wiwa]

Instead of ascribing

val x: Foo[X] forSome { type X }

we will ascribe

val x: Foo[_]

[ShaneDelmore]

This is an old ticket and we don't have a reproduction for the issue or know if it is still valid so I am closing it for now.

[wiwa]

Given:

class Box[A]

def foo = null.asInstanceOf[Box[_]]

Obtained:

def foo: Box[T1] forSome { type T1 } = null.asInstanceOf[Box[_]]

Expected:

def foo: Box[_] = null.asInstanceOf[Box[_]]

Relates to #471 and requires Box to be invariant.