Roslyn symbol resolution fails after unloading and re-loading an SDK-style project.

In VS2017 15.9.11, I use the shell interfaces to programmatically unload and reload a C# project, and after the reload, Roslyn symbol resolution does not work. Inserting a delay (of up to 60 seconds) doesn't help.

Interestingly, this behavior is NOT observed with a non-SDK-style project. After unloading and re-loading the project, Roslyn is able to immediately resolve symbols.

The following project is all that is needed to see the failure:

```

net45

```

Use this code to resolve/unload/reload/resolve again:

```
var componentModel = (IComponentModel)serviceProvider.GetService(typeof(SComponentModel));

var workspace = componentModel.GetService();

var project = workspace.CurrentSolution.Projects.Single();

var compilation = ThreadHelper.JoinableTaskFactory.Run(async () => await project.GetCompilationAsync());

// Symbol resolves here.
//
var symbol = compilation.GetSpecialType(SpecialType.System_Int32);

var solution = (IVsSolution)serviceProvider.GetService(typeof(SVsSolution));

solution.GetProjectOfUniqueName(project.FilePath, out var hierarchy);

var projectId = solution.GetProjectIdGuid(hierarchy);

solution.UnloadProject(hierarchy);

solution.ReloadProject(projectId);

project = workspace.CurrentSolution.Projects.Single();

compilation = ThreadHelper.JoinableTaskFactory.Run(async () => await project.GetCompilationAsync());

// This returns null.
//
var symbol = compilation.GetSpecialType(SpecialType.System_Int32);
```

If you repeat the above on a from-scratch C# application that is NOT SDK-style, then you will see the above symbol resolution work just fine -- before and after the re-load.

It "feels" like Roslyn is waiting for something to "kick" it after an unload/reload cycle.

Very interested in this because we have a critical need to be able to unload a project do some work on the project file, reload it, and do some Roslyn transforms.

Thanks very much in advance.