Generate static backing fields for properties in object

#KT-4973 Fixed

compiler/testData/codegen/box/staticFields/syntheticAccessor.kt

@@ -0,0 +1,12 @@
+object A {
+    private val p = "OK";
+
+    object B {
+        val z = p;
+    }
+
+}
+
+fun box(): String {
+    return A.B.z
+}