Create SyntheticMemberDescriptor interface

Implement this interface by sam adapters/constructors and use it in navigation

 #KT-11708 Fixed

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/LookupElementFactory.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2015 JetBrains s.r.o.
+ * Copyright 2010-2016 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -305,7 +305,7 @@ class LookupElementFactory(
             return callableWeight(descriptor.getMethod)
         }
         if (descriptor is SamAdapterExtensionFunctionDescriptor) {
-            return callableWeight(descriptor.sourceFunction)
+            return callableWeight(descriptor.baseDescriptorForSynthetic)
         }
 
         if (descriptor.overriddenDescriptors.isNotEmpty()) {