Make Throwable properties message and cause open.

#KT-5587 Fixed

compiler/testData/codegen/box/specialBuiltins/throwableImpl.kt

@@ -0,0 +1,21 @@
+class MyThrowable(message: String? = null, cause: Throwable? = null) : Throwable(message, cause) {
+
+    override val message: String?
+        get() = "My message: " + super.message
+
+    override val cause: Throwable?
+        get() = super.cause ?: this
+
+}
+
+fun box(): String {
+    try {
+        throw MyThrowable("test")
+    } catch (t: MyThrowable) {
+        if (t.cause != t) return "fail t.cause"
+        if (t.message != "My message: test") return "fail t.message"
+        return "OK"
+    }
+
+    return "fail: MyThrowable wasn't catched."
+}