roslyn: Cannot rename named tuples

(Updated - jcouv: a more detailed spec-in-progress below)

Steps to Reproduce:

1. Create a new C# project
2. Have some code like:

var tuple = (number: 5, greeting: "Hello");
var number = tuple.number;
var greeting = tuple.greeting;

1. select greeting in tuple.greeting
2. attempt to rename greeting

Expected Behavior: Inline rename offered for tuple.greeting

Actual Behavior: