FindAllReferences should handle Dispose referenced in using and foreach statements

[jcouv]

See comment in ComputeImplicitReferenceConflictsAsync: // TODO: handle Dispose for using statement and Add methods for collection initializers.

Note, if this done at the same time or after C# 8.0, FAR should also account for DisposeAsync in async-foreach and async-dispose statements.

Relates to #28230

[jcouv]

We should also handle the using declaration statement and the asynchronous variants of using statements and declarations.

I'm expecting Dispose and DisposeAsync to be referenced

using System;
using System.Threading.Tasks;

class C : IAsyncDisposable

{
    async Task M()
    {
        await using (var x = new C())
        {
        }

        await using var x2 = new C();
    }
    public ValueTask DisposeAsync() => throw new NotImplementedException();
}

class C2 : IDisposable
{
    void M()
    {
        using (var x = new C2())
        {
        }

        using var x2 = new C2();

    }
    public void Dispose() => throw new NotImplementedException();
}

[CyrusNajmabadi]

@jcouv is there an api the compiler exposes to see what Dispose method a using binds to?

[CyrusNajmabadi]

Dupe of #34107