Fixed bug in default args lowering + test

Made proper overridden descriptor selection.

backend.native/tests/build.gradle

@@ -349,6 +349,11 @@ task function_defaults10(type: RunKonanTest) {
     source = "codegen/function/defaults10.kt"
 }
 
+task function_defaults_from_fake_override(type: RunKonanTest) {
+    goldValue = "42\n"
+    source = "codegen/function/defaultsFromFakeOverride.kt"
+}
+
 task sum_3const(type: RunKonanTest) {
     source = "codegen/function/sum_3const.kt"
 }

backend.native/tests/codegen/function/defaultsFromFakeOverride.kt

@@ -0,0 +1,18 @@
+package codegen.function.defaultsFromFakeOverride
+
+import kotlin.test.*
+
+interface I<T> {
+    fun f(x: String = "42"): String
+}
+
+open class A<T> {
+    open fun f(x: String) = x
+}
+
+class B : A<String>(), I<String>
+
+@Test fun runTest() {
+    val b = B()
+    println(b.f())
+}