Champion "conditional ref operator" (C# 7.2)

- [x] Proposal added
(https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/conditional-ref.md)
- [x] Discussed in LDM
- [x] Decision in LDM
- [x] Finalized (done, rejected, inactive)
- [ ] Spec'ed

See also
- https://github.com/dotnet/roslyn/issues/16473 (original spec)
- https://github.com/dotnet/roslyn/pull/17436 (prototype)
- The most up-to-date description of the feature (as implemented in C# 7.2) is a part of the following document:
https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.2/readonly-ref.md

The idea would be to support a version of the `?:` operator that yields an lvalue. The current proposal is to do so using `ref` after the `:` and `?`, for example

``` c#
(array1 != null ? ref array1[index] : ref dummy) = value;
```

``` c#
ref Node next = ref (node != null) ? ref node.Next : ref head;
```