nameof can refer to a variable "early"

Spotted in #608 by @KalleOlaviNiemitalo, but split off as it's not part of the change.

In statements.md:

> The scope of a local constant is the block in which the declaration occurs. It is an error to refer to a local constant in a textual position that precedes the end of its *constant_declarator*.

But `const string s = nameof(s);` is fine, despite the reference in `nameof(s)` being before the end of the declarator.

It's possible that we just need a carve-out for nameof in this case.