[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/codegen/box/jvm8/defaults/kt11969.kt

@@ -1,6 +1,5 @@
 // !JVM_DEFAULT_MODE: enable
 // IGNORE_BACKEND_FIR: JVM_IR
-// IGNORE_BACKEND: JVM_IR
 // TARGET_BACKEND: JVM
 // JVM_TARGET: 1.8
 // WITH_RUNTIME