psi2ir: Don't generate constructor bodies for 'external' classes

#KT-27934

compiler/testData/ir/irJsText/native/nativeNativeKotlin.kt

@@ -0,0 +1,39 @@
+// FILE: nativeNativeKotlin.kt
+
+package foo
+
+external open class A {
+    fun foo(): String
+}
+
+external open class B : A {
+    fun bar(): String
+}
+
+class C : B()
+
+fun box(): String {
+    val c = C()
+    return "OK"
+}
+
+// FILE: nativeNativeKotlin.js
+
+function A() {
+
+}
+
+A.prototype.foo = function () {
+    return "A.foo"
+};
+
+function B() {
+
+}
+
+B.prototype = Object.create(A.prototype);
+B.prototype.constructor = B;
+
+B.prototype.bar = function () {
+    return "B.bar"
+};