[JVM IR] Leave private @JvmDefault methods on interface

Like the old backend, always leave private @JvmDefault annotated
interface members (properties, methods) on the interface, just like
the old backend. Fix naming, and introduce test to document the naming
scheme.

compiler/testData/writeFlags/jvm8/defaults/privateAccessorNaming.kt

@@ -0,0 +1,18 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// TARGET_BACKEND: JVM
+// !JVM_DEFAULT_MODE: enable
+// JVM_TARGET: 1.8
+// WITH_RUNTIME
+interface I {
+
+    @JvmDefault
+    private fun foo() = 4
+
+    fun bar() = { foo() + 5 }()
+
+}
+
+
+// TESTED_OBJECT_KIND: function
+// TESTED_OBJECTS: I, access$foo
+// FLAGS: ACC_PUBLIC, ACC_STATIC, ACC_SYNTHETIC