KT-6191 Support completion for backing field

#KT-6191 Fixed

idea/testData/completion/smart/BackingFields.kt

@@ -0,0 +1,27 @@
+class C {
+    var property1 = "abc" // has backing field but accessors are default - no sense to show in completion
+
+    var property2: String // no backing field
+        get() = "abc"
+        set(value){}
+
+    var property3 = "abc" // has backing field and accessor
+        get() = $property3 + 1
+
+    var property4 = 1
+        get() = $property4 + 1
+
+    var property5: String? = null
+        get() = $property3 + 1
+
+    fun foo(): String {
+        return <caret>
+    }
+}
+
+// ABSENT: $property1
+// ABSENT: $property2
+// EXIST: $property3
+// ABSENT: $property4
+// EXIST: { itemText: "!! $property5" }
+// EXIST: { itemText: "?: $property5" }