[JS IR BE] Support companion objects in external interfaces

These objects are referenced but value is not actually used.
Replace access to these references with null literals.

js/js.translator/testData/box/regression/companionObjectInExternalInterface.kt

@@ -0,0 +1,17 @@
+// EXPECTED_REACHABLE_NODES: 1303
+
+// This hack is used in org.w3c.* part of standard library to represent unions of Strings
+// Test that we are not actually trying to access nonexistent companion object
+
+@Suppress("NESTED_CLASS_IN_EXTERNAL_INTERFACE")
+public external interface I {
+    companion object
+}
+public inline val I.Companion.O: I get() = "O".asDynamic().unsafeCast<I>()
+public inline val I.Companion.K: I get() = "K".asDynamic().unsafeCast<I>()
+
+fun box(): String {
+    if (I.O != I.O) return "Fail 1"
+    if (I.O == I.K) return "Fail 2"
+    return I.O.unsafeCast<String>() + I.K.unsafeCast<String>()
+}