JVM IR: Do not generate DefaultImpls if it's empty. This revealed that InterfaceDelegationLowering was relying on the presence of DefaultImpls even when empty. Hence, simply dropping it from InterfaceLowering was not trivial. Moved default delegation in DefaultImpls to SuperType.DefaultImpls to InterfaceLowering. Clean up logic considerably, and document inter-phase dependencies.

compiler/testData/writeFlags/jvm8/defaults/compatibility/defaultImplementations.kt

@@ -0,0 +1,22 @@
+// !JVM_DEFAULT_MODE: compatibility
+// JVM_TARGET: 1.8
+// WITH_RUNTIME
+
+interface A {
+    @JvmDefault
+    fun foo(x: Int = 0): Int {
+        return x
+    }
+}
+
+// TESTED_OBJECT_KIND: innerClass
+// TESTED_OBJECTS: A, DefaultImpls
+// FLAGS: ACC_FINAL, ACC_STATIC, ACC_PUBLIC
+
+// TESTED_OBJECT_KIND: function
+// TESTED_OBJECTS: A, foo$default
+// FLAGS: ACC_PUBLIC, ACC_SYNTHETIC,  ACC_STATIC
+
+// TESTED_OBJECT_KIND: function
+// TESTED_OBJECTS: A$DefaultImpls, foo$default
+// FLAGS: ACC_PUBLIC, ACC_SYNTHETIC, ACC_STATIC