Follow up from #85911

Background and motivation

This proposal is about also updating Unsafe.AsPointer to use ref readonly. This is the same change we did for the other APIs in #85911, and it is not breaking. We didn't include AsPointer at the time as we didn't have any use cases. This would now benefit us in CsWinRT (see #114024), as it would simplify all property accessors to get the CCW vtables for projected and custom mapped types.

API Proposal

namespace System.Runtime.CompilerServices
{
    public static class Unsafe
    {
-       public static void* AsPointer<T>(ref T value);
+       public static void* AsPointer<T>(ref readonly T value);
    }
}

API Usage

public static class SomeTypeImpl
{
    [FixedAddressValueType];
    private static readonly SomeTypeVtbl Vftbl;

    public static nint Vftbl => (nint)Unsafe.AsPointer(in Vftbl);

    static SomeTypeImpl()
    {
        // Initialize vtable
    }
}

Risks

None. Unsafe.AsPointer is already an unsafe API, and it's in an unsafe namespace. This API makes things simpler, but developer could already achieve the same, just by adding an Unsafe.AsRef call on top of this chain. Just updating the signature for AsPointer simplifies all callsites instead.