Add reference projects for new Roslyn dependencies

[JoeRobich]

As mentioned in dotnet/dotnet#2842 (comment), reference projects need to be generated for the following packages.

Microsoft.Build,17.11.31
Microsoft.Build.Framework,17.11.31
Microsoft.Build.Locator,1.10.2
Microsoft.Build.Tasks.Core,17.11.31
Microsoft.Build.Utilities.Core,17.11.31
Microsoft.NET.StringTools,17.11.31
System.Reflection.MetadataLoadContext,8.0.0

[ViktorHofer]

Just a reminder that this also needs to go into release/10.0