Null-conditional operator spec bug: E0 definition doesn't consider nullable value types

From https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#null-conditional-operator

> For a null_conditional_expression E with a primary_expression P, let E0 be the expression obtained by textually removing the leading ? from each of the null_conditional_operations of E that have one.

Later, T0 is defined as the type of E0.

This seems to fall short in the case of nullable value types:

```
var d = (DateTime?)DateTime.Now;
var y = d?.Year; // valid
```
E0 according to the spec would be `d.Year`, which is not valid. What is T0 in this case?

https://github.com/dotnet/csharplang/blob/master/meetings/2014/LDM-2014-02-03.md#semantics clarified this detail explicitly:

> Where e0 is the same as e, except if e is of a nullable value type, in which case e0 is e.Value.

But that wording did not seem to make it into the spec.