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" }

idea/testData/completion/basic/common/BackingFields2.kt

@@ -0,0 +1,4 @@
+var globalProperty = "abc"
+    get() = <caret>
+
+// EXIST: $globalProperty

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

idea/testData/completion/basic/common/BackingFields4.kt

@@ -0,0 +1,10 @@
+class C {
+    var property = "abc"
+        get() = $property + 1
+
+    fun foo() {
+        this.<caret>
+    }
+}
+
+// EXIST: $property

idea/testData/completion/basic/common/BackingFields5.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

idea/testData/completion/basic/common/BackingFields6.kt

@@ -0,0 +1,13 @@
+class C {
+    var aProperty = "abc"
+        get() {}
+    var bProperty = "abc"
+        get() {}
+
+    fun foo(){
+        $a<caret>
+    }
+}
+
+// EXIST: $aProperty
+// ABSENT: $bProperty

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