Add a test to ensure that erased classes are actually erased at compile time

Author: natsukagami

tests/run-custom-args/erased/erased-class-are-erased.check

@@ -0,0 +1 @@
+27

tests/run-custom-args/erased/erased-class-are-erased.scala

@@ -0,0 +1,14 @@
+object Test {
+  erased class Erased() {
+    println("Oh no!!!")
+  }
+
+  def f(x: Erased, y: Int = 0): Int = y + 5
+
+  def g() = Erased()
+
+  def main(args: Array[String]) =
+    val y = Erased()
+    val z = 10
+    println(f(Erased()) + z + f(g(), 7))
+}