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

KT-65687

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

@@ -0,0 +1,7 @@
+private interface PrivateA {
+    fun foo()
+}
+
+class PublicB : PrivateA {
+    override fun foo() = Unit
+}