Fix casts of Unit value to other types

compiler/testData/codegen/boxWithStdlib/casts/unitAsInt.kt

@@ -0,0 +1,15 @@
+fun foo() {}
+
+fun box(): String {
+    try {
+        foo() as Int?
+    }
+    catch (e: ClassCastException) {
+        return "OK"
+    }
+    catch (e: Throwable) {
+        return "Fail: ClassCastException should have been thrown, but was instead ${e.javaClass.getName()}: ${e.getMessage()}"
+    }
+
+    return "Fail: no exception was thrown"
+}