Generate delegation body for default interface methods, calculate proper abstractness for them

compiler/testData/codegen/java8/writeFlags/defaultMethod.kt

@@ -0,0 +1,16 @@
+// KOTLIN_CONFIGURATION_FLAGS: +JVM.JVM_8_TARGET
+interface Test {
+    fun test(): String {
+        return "OK"
+    }
+
+    fun testAbstract(): String
+}
+
+// TESTED_OBJECT_KIND: function
+// TESTED_OBJECTS: Test, test
+// FLAGS: ACC_PUBLIC
+
+// TESTED_OBJECT_KIND: function
+// TESTED_OBJECTS: Test, testAbstract
+// FLAGS: ACC_PUBLIC, ACC_ABSTRACT