[AllOpen] Private properties can be considered stable even when open

The AllOpen plugin can make private members open. But for private
properties, they can be considered stable for smart-casting if they do
not have a custom getter.

#KT-58049 Fixed

plugins/allopen/testData/diagnostics/smartcast.kt

@@ -0,0 +1,24 @@
+// WITH_STDLIB
+// ISSUE: KT-58049
+
+annotation class AllOpen
+
+@AllOpen
+class Test(
+    val publicProp: String?,
+    protected val protectedProp: String?,
+    internal val internalProp: String?,
+    private val privateProp: String?,
+) {
+    fun test() {
+        checkNotNull(publicProp)
+        checkNotNull(protectedProp)
+        checkNotNull(internalProp)
+        checkNotNull(privateProp)
+
+        println(<!SMARTCAST_IMPOSSIBLE!>publicProp<!>.length)
+        println(<!SMARTCAST_IMPOSSIBLE!>protectedProp<!>.length)
+        println(<!SMARTCAST_IMPOSSIBLE!>internalProp<!>.length)
+        println(<!DEBUG_INFO_SMARTCAST!>privateProp<!>.length)
+    }
+}