Generate static backing fields for properties in object

#KT-4973 Fixed

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

@@ -0,0 +1,13 @@
+class Foo {
+    class object {
+        val bar: String
+
+        {
+            bar = "OK"
+        }
+    }
+}
+
+fun box(): String {
+    return Foo.bar
+}