SI-6385 Avoid bridges to identical signatures over value classes

As Paul noted in the comments to SI-6260 (from which I mined some
test cases) "there is no possible basis for conflict here":

    scala> class C[A](val a: Any) extends AnyVal
    defined class C

    scala> class B { def x[A](ca: C[A]) = () }
    defined class B

    scala> class D extends B { override def x[A](ca: C[A]) = () }
    <console>:8: error: bridge generated for member method x: [A](ca: C[A])Unit in class D
    which overrides method x: [A](ca: C[A])Unit in class B
    clashes with definition of the member itself;
    both have erased type (ca: Object)Unit
           class D extends B { override def x[A](ca: C[A]) = () }
                                            ^
What was happening?

Bridge computation compares `B#x` and `D#x` exitingErasure, which
results in comparing:

    ErasedValueType(C[A(in B#x)]) =:= ErasedValueType(C[A(in D#x)])

These types were considered distinct (on the grounds of the unique
type hash consing), even though they have the same erasure and
involve the same value class.

That triggered creation of an bridge. After post-erasure eliminates
the `ErasedValuedType`s, we find that this marvel of enginineering is
bridges `(Object)Unit` right back onto itself. The previous
resolution of SI-6385 (d435f72e5fb7fe) was a test case that confirmed
that we detected the zero-length bridge and reported it nicely, which
happened after related work in SI-6260. But we can simply avoid
creating in it in the first place.

That's what this commit does. It does so by reducing the amount
of information carried in `ErasedValueType` to the bare minimum needed
during the erasure -> posterasure transition.

We need to know:

  1. which value class wraps the value, so we can box and unbox
     as needed
  2. the erasure of the underlying value, which will replace this
     type in post-erasure.

This construction means that the bridge above computation now
compares:

   ErasedValueType(C, Any) =:= ErasedValueType(C, Any])

I have included a test to show that:

  - we don't incur any linkage or other runtime errors in the
    reported case (run/t6385.scala)
  - a similar case compiles when the signatures align
    (pos/t6260a.scala), but does *not* compile when the just
    erasures align (neg/t6260c.scala)
  - polymorphic value classes continue to erase to the instantiated
    type of the unbox: (run/t6260b.scala)
  - other cases in SI-6260 remains unsolved and indeed unsolvable
    without an overhaul of value classes: (neg/t6260b.scala)

In my travels I spotted a bug in corner case of null, asInstanceOf
and value classes, which I have described in a pending test.