Remove constraints on stackalloc (support new locations)

[gafter]

No description provided.

[gafter]

This change reflects the following behavior.

This is illegal:

    public void M() {
        var p = stackalloc nint[3];
    }

Because a stackalloc expression used as an initializer is of type T* (an unsafe pointer type) which requires an unsafe context. However, these are legal:

    public void M() {
        Span<nint> p = stackalloc nint[3];
    }

and

    public void M() {
        var p = (stackalloc nint[3]);
    }

(Note the addition of parens)

[gafter]

I don't understand the following behavior:

This is illegal:

    public void M() {
        var p = stackalloc nint[3][1];
    }

I don't understand why. Perhaps because `nint[3] is parsed as a type. I get the error

•	error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type ('nint[]')
•	error CS1575: A stackalloc expression requires [] after type

Yet this is legal:

    public void M() {
        var p = (stackalloc nint[3])[1];
    }