Correctly synthesize manifest[T] when T is an alias

[lrytz]

This

class A[T]
object A {
  type T = A[_]
  manifest[T]
}

crashed the compiler. Comparing the AST generated for manifest[T] with
the working version manifest[A[_]] shows that a cast is inserted to
the classOf argument in the latter case, but not the former.

For manifest[T]:

scala.Predef.manifest[A.T](
  scala.reflect.ManifestFactory.classType[A.T](
    classOf[A],
    ...

For manifest[A[_]]

scala.Predef.manifest[A[_]](
  scala.reflect.ManifestFactory.classType[A[_]](
    classOf[A].asInstanceOf[Class[A[_]]],
    ...

My approach for fixing this was simply to see what makes the compiler
insert the cast. The condition is here:

private def manifestOfType(tp: Type, flavor: Symbol): SearchResult = {
  ...
  def mot(tp0: Type, from: List[Symbol], to: List[Type]): SearchResult = {
    val tp1 = tp0.dealias
    ...
    val classarg = tp.dealias match {
      case _: ExistentialType => gen.mkCast(classarg0, ClassType(tp))
      case _                  => classarg0
    }

tp is A.T, the type alias. In the first call to mot, tp0 is
A.T. tp1 = tp0.dealias is an ExistentialType, so mot called
recursively with tp0 = tp1.skolemizeExistential.

A cast seems to be needed if the original type tp is an existential
(not sure why that is), but we need to dealias. Not sure if we should
cast to tp or tp.dealias, I guess it doesn't matter.

Fixes scala/bug#9155

[lrytz]

@adriaanm updated the commit message.

[adriaanm]

💯