Generate private interface companion object as package-private synthetic

compiler/testData/codegen/bytecodeListing/companionObjectVisibility_after.kt

@@ -0,0 +1,17 @@
+// !LANGUAGE: +ProperVisibilityForCompanionObjectInstanceField
+
+open class TestProtectedCompanionInClass {
+    protected companion object
+}
+
+class TestInternalCompanionInClass {
+    internal companion object
+}
+
+class TestPrivateCompanionInClass {
+    private companion object
+}
+
+interface TestPrivateCompanionInInterface {
+    private companion object
+}