KT-36973 Keep private default interface members private

compiler/testData/codegen/box/traits/kt36973.kt

@@ -0,0 +1,15 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// FILE: kt36973.kt
+import other.*
+
+class C : IFoo
+
+fun box() = C().foo()()
+
+// FILE: IFoo.kt
+package other
+
+interface IFoo {
+    fun foo() = { bar() }
+    private fun bar() = "OK"
+}

compiler/testData/codegen/bytecodeListing/privateDefaultImpls.kt

@@ -0,0 +1,4 @@
+interface A {
+    private fun foo() {}
+    private val x get() = 1
+}

compiler/testData/codegen/bytecodeListing/privateDefaultImpls.txt

@@ -0,0 +1,11 @@
+@kotlin.Metadata
+public final class A$DefaultImpls {
+    inner class A$DefaultImpls
+    private static method foo(p0: A): void
+    private static method getX(p0: A): int
+}
+
+@kotlin.Metadata
+public interface A {
+    inner class A$DefaultImpls
+}