Code completion: do not list instance members from context with no instance

#KT-6388 Fixed
 #KT-4422 Fixed
 #KT-5516 Fixed

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

@@ -0,0 +1,25 @@
+class C {
+    val field = 0
+    class NestedClass
+    inner class InnerClass
+    object AnObject
+
+    class object {
+        val classObjectField = 0
+        class ClassObjectClass
+
+        fun foo(){
+            <caret>
+        }
+    }
+}
+
+fun C.extensionForC(){}
+
+// ABSENT: field
+// EXIST: NestedClass
+// EXIST: InnerClass
+// EXIST: AnObject
+// EXIST: classObjectField
+// EXIST: ClassObjectClass
+// ABSENT: extensionForC