Calls to extension properties for `readonly struct` treat setters as mutating the foreach loop variable

**Version Used**: 10.0.102

**Steps to Reproduce**:

1. Attempt to compile the following listing:

```cs
using System;
using System.Collections.Generic;

public class Api
{
public int GetSomeProper(int index)
=> throw new NotImplementedException();

public void SetSomeProper(int index, int value)
=> throw new NotImplementedException();
}

public readonly struct Blah(Api api, int index)
{
public readonly Api Api = api;
public readonly int Index = index;
}

public static class BlahExtensions
{
extension(in Blah blah) // Please note that `in` param doesn't make a difference
{
public int SomeProp
{
get => blah.Api.GetSomeProper(blah.Index);
set => blah.Api.SetSomeProper(blah.Index, value);
}
}
}

public static class Program
{
public static void Main()
{
var api = new Api();
var lst = new List();
lst.Add(new Blah(api, 1));
lst.Add(new Blah(api, 2));

foreach (var b in lst)
{
b.SomeProp = 123; // doesn't compile - cannot modify the loop variable inside foreach
BlahExtensions.set_SomeProp(b, 123); // Compiles
}
}
}
```

2. Receive error message:

```
Program.cs(42,13): Error CS1654 : Cannot modify members of 'b' because it is a 'foreach iteration variable'
```

**Diagnostic Id**: CS1654

**Expected Behavior**: Listing compiles

**Actual Behavior**: Doesn't compile, reporting the CS1654 compile time error.