KT-6191 Support completion for backing field

#KT-6191 Fixed

idea/testData/completion/basic/common/BackingFields3.kt

@@ -0,0 +1,11 @@
+class C {
+    var property = "abc"
+        get() = $property + 1
+}
+
+
+fun foo(c: C){
+    c.<caret>
+}
+
+// ABSENT: $property