Fix CANNOT_CHECK_FOR_ERASED for when()

compiler/testData/diagnostics/tests/cast/WhenWithExpression.kt

@@ -0,0 +1,9 @@
+enum class List<out T>(val size : Int) {
+    Nil : List<Nothing>(0)
+}
+
+fun List<String>.join() =
+        when (this) {
+            List.Nil -> "[]" // CANNOT_CHECK_FOR_ERASED was reported
+            else -> ""
+        }