KT-6191 Support completion for backing field

#KT-6191 Fixed

idea/testData/completion/basic/common/BackingFields1.kt

@@ -0,0 +1,26 @@
+var globalProperty = "abc" // has backing field and accessor but backing field is not accessible
+    get() = $globalProperty + 1
+
+class C {
+    var property1 = "abc" // has backing field but accessors are default - no sense to show in completion
+
+    var property2 = "abc" // has backing field but accessors are also default
+        private set
+
+    var property3: String // no backing field
+        get() = "abc"
+        set(value){}
+
+    var property4 = "abc" // has backing field and accessor
+        get() = $property4 + 1
+
+    fun foo(){
+        <caret>
+    }
+}
+
+// ABSENT: $globalProperty
+// ABSENT: $property1
+// ABSENT: $property2
+// ABSENT: $property3
+// EXIST: { lookupString: "$property4", itemText: "$property4", tailText: null, typeText: "String" }