23.4 Fixed and moveable variables

The language spec (23.4 Fixed and moveable variables) says

> In precise terms, a fixed variable is one of the following:
> - A variable resulting from a simple-name (§7.6.2) that refers to a local variable or a value parameter, unless the variable is captured by an anonymous function.
> - A variable resulting from a member-access (§7.6.4) of the form V.I, where V is a fixed variable of a struct-type.
> - A variable resulting from a pointer-indirection-expression (§18.5.1) of the form *P, a pointer-member-access (§18.5.2) of the form P->I, or a pointer-element-access (§18.5.3) of the form P[E].
>
> All other variables are classified as moveable variables.

But testing this on the following program

```csharp
using System;
unsafe struct S
{
public fixed int buffer[1];
public int i;
}

unsafe class Test
{
private void example1()
{
S data = new S();
Func lambda = () => data;
fixed (int* p = &data.i) // ok; data is captured
{
}
fixed (int* p = data.buffer) // ok; data is captured
{
}
}
}
```

We find that both the native compiler and Roslyn reject both attempts, complaining that you can't take the address of a variable that has been captured by a lambda, AND that you can't use the fixed statement to take the address of an already fixed expression.

We should probably modify the language spec to agree with the implementation.