MSB4011 is not suppressable by MSBuildWarningsAsMessages

Originally reported at https://developercommunity.visualstudio.com/content/problem/400893/using-msbuildwarningsasmessages-in-directorybuildp.html.

> I have added Directory.Build.props to the same folder as my solution with content:
>
> ```xml
>
>
> MSB4011
>
>
> ```
>
> The problem is that I still get the MSB4011 warnings when I compile.
>
> I know that file is found because if I change the `` tag to ``, I get an error when compiling.
>
> Why isn't it working?
>
> I am using visual studio professional 2017, 15.9.3. The projects are C++.