[JS IR BE] Simplify throwable lowering.

- Make kotlin.Throwable class extenral
- Add runtime function 'extendThrowable' to use instead of delegating
  constructors to Throwable

libraries/stdlib/js-ir/builtins/Throwable.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin
+
+/**
+ * The base class for all errors and exceptions. Only instances of this class can be thrown or caught.
+ *
+ * @param message the detail message string.
+ * @param cause the cause of this throwable.
+ */
+@JsName("Error")
+public external open class Throwable {
+    open val message: String?
+    open val cause: Throwable?
+
+    constructor(message: String?, cause: Throwable?)
+    constructor(message: String?)
+    constructor(cause: Throwable?)
+    constructor()
+}