[ObjCType] Add basic implementation of base method, fix tests

KT-65687

native/objcexport-header-generator/testData/headers/privateGenericSuperInterface/Foo.kt

@@ -0,0 +1,7 @@
+private interface PrivateInterface<T> {
+    fun foo(): T
+}
+
+class PublicClass : PrivateInterface<Int> {
+    override fun foo(): Int = error("stub")
+}