roslyn: The project context navigation bar doesn't cause classification to update.

Short project setup (internal only):

1. Copy the Closed\Test\Files\SharedProjectCS directory from our internal sources to another location (e.g., Desktop.)
2. Open TestProj.sln.
3. Open Class2.cs from the shared project.

Long project setup (manual):

1. Create a new Windows app.
2. Add a new WindowsPhone app to the solution.
3. Add a shared project to the solution that both Windows and WindowsPhone apps reference.
4. Add a file to the shared project with the body:

namespace TestNamespace
{
    public class TestClass
    {
        #if WINDOWS_PHONE_APP
        public void PhoneMethod(bool b) { }
        #elif WINDOWS_APP
        public void WinMethod(int i) { }
        #endif
    }
}

Rest of the repro steps:

1. With the shared file open, verify that changing the project context navigation bar causes the classification inside the #if/#elif/#endif block to change.
2. Close the shared file.
3. Re-open the shared file.
4. Change the project context navigation bar.

Result: The classification no longer changes.

(This failure is visible in the internal-only CSharpSharedProjects integration test.)