Clearer message that running .NET Framework MSBuild in a .NET process is unsupported

[maridematte]

Originating from #10175 discussion.

The scenario of running .NET Framework MSBuild in a .NET process is unsupported, but it worked until a recent change. Since there is some confusion around this, identifying and breaking this scenario with a specific message would be ideal.

[rainersigwald]

A wrinkle here is that our API has a lot of entry points. However an exception in the evaluator would probably catch almost everything.

[KalleOlaviNiemitalo]

Can ModuleInitializerAttribute be used for this? It would cause the exception to be wrapped in TypeInitializationException, though.

.NET Framework does not provide the System.Runtime.CompilerServices.ModuleInitializerAttribute type, but if you define a similar one as internal in MSBuild, then I think Roslyn will recognize it and set up the <Module>::.cctor method that the runtime will then call.