KT-6191 Support completion for backing field

#KT-6191 Fixed

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

@@ -0,0 +1,15 @@
+class C {
+    var property1 = "abc"
+        get() = $property1 + 1
+    var property2 = "abc"
+        get() = $property2 + 1
+
+    fun foo(){
+        "${$<caret>}"
+    }
+}
+
+// EXIST: $property1
+// EXIST: $property2
+// ABSENT: property1
+// ABSENT: property2