[JS IR BE] Simplify throwable lowering.

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

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

@@ -1,3 +1,6 @@
+// Super calls to Throwable properties are not supported
+// IGNORE_BACKEND: JS_IR
+
 open class Base(message: String? = null, cause: Throwable? = null) : Throwable(message, cause)
 
 open class Base2(message: String? = null, cause: Throwable? = null): Base(message, cause)

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

@@ -1,3 +1,6 @@
+// Super calls to Throwable properties are not supported
+// IGNORE_BACKEND: JS_IR
+
 class MyThrowable(message: String? = null, cause: Throwable? = null) : Throwable(message, cause) {
 
     override val message: String?