Consider adjusting wording for ErrorCode.ERR_NoConvToIDisp

[AlekseyTs]

        [Fact]
        public void Using_06()
        {
            var src = @"
interface IMyDisposable
{
    void Dispose();
}

ref struct S2 : IMyDisposable
{
    public void Dispose()
    {
    }
}

class C
{
    static void Main()
    {
        Test(new S2(), null);
    }

    static void Test<T>(T t, IMyDisposable s) where T : IMyDisposable, allows ref struct
    {
        using (t)
        {
        }

        using (s)
        {
        }
    }
}
";
            var comp = CreateCompilation(src, targetFramework: s_targetFrameworkSupportingByRefLikeGenerics, options: TestOptions.ReleaseExe);

            // PROTOTYPE(RefStructInterfaces): Should we adjust wording for the error? Ref struct implement interfaces, but not convertible to them.  
            comp.VerifyDiagnostics(
                // (23,16): error CS1674: 'T': type used in a using statement must be implicitly convertible to 'System.IDisposable'.
                //         using (t)
                Diagnostic(ErrorCode.ERR_NoConvToIDisp, "t").WithArguments("T").WithLocation(23, 16),
                // (27,16): error CS1674: 'IMyDisposable': type used in a using statement must be implicitly convertible to 'System.IDisposable'.
                //         using (s)
                Diagnostic(ErrorCode.ERR_NoConvToIDisp, "s").WithArguments("IMyDisposable").WithLocation(27, 16)
                );
        }

Observed: See PROTOTYPE comment in the test