Add tests for scala#4060 and small change in i11896

For i11896, we now no longer allow an erased val to be of a
top-level type declaration X (since it is not a concrete type,
per realizability rules). `class X` is used instead.

Another test, `top-level-type`, was added to check this behaviour.

Author: natsukagami

tests/neg-custom-args/erased/i4060.scala

@@ -0,0 +1,14 @@
+// See https://github.com/lampepfl/dotty/issues/4060#issuecomment-445808377
+
+object App {
+  trait A { type L >: Any}
+  //def upcast(a: A, x: Any): a.L = x
+  def upcast(erased a: A)(x: Any): a.L = x
+  //lazy val p: A { type L <: Nothing } = p
+  erased val p: A { type L <: Nothing } = p // error
+  def coerce(x: Any): Int = upcast(p)(x)
+
+  def main(args: Array[String]): Unit = {
+    println(coerce("Uh oh!"))
+  }
+}

tests/neg-custom-args/erased/top-level-type.scala

@@ -0,0 +1,4 @@
+import scala.language.experimental.erasedDefinitions
+
+type X
+erased def x : X = compiletime.erasedValue // error

tests/pos-custom-args/erased/i11896.scala

@@ -1,6 +1,6 @@
 import scala.language.experimental.erasedDefinitions
 
-type X
+class X
 erased def x: X = compiletime.erasedValue
 
 def foo(using erased X): Unit = ()