[JS IR BE] Make kotlinx.io compile

- Fix expect/actual default arguments
 - Fix dynamic type in inliner
 - Fix external varargs

compiler/testData/codegen/box/multiplatform/defaultArguments/inheritedFromExpectedFunction.kt

@@ -0,0 +1,17 @@
+// !LANGUAGE: +MultiPlatformProjects
+// WITH_RUNTIME
+// MODULE: lib
+// FILE: common.kt
+
+expect fun foo(a: String, b: String = "O"): String
+
+// FILE: platform.kt
+
+actual fun foo(a: String, b: String) = a + b
+
+// MODULE: main(lib)
+// FILE: main.kt
+
+fun box(): String {
+    return foo("") + foo("K", "")
+}