Switch a call to LanguageServices.GetService to GetRequiredService

If we're going to require it, let's at least crash with a useful
exception instead of a null ref.

src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.CompilationTracker.cs

@@ -502,7 +502,7 @@ private async Task<Compilation> GetOrBuildDeclarationCompilationAsync(SolutionSt
 
             private Compilation CreateEmptyCompilation()
             {
-                var compilationFactory = this.ProjectState.LanguageServices.GetService<ICompilationFactoryService>();
+                var compilationFactory = this.ProjectState.LanguageServices.GetRequiredService<ICompilationFactoryService>();
 
                 if (this.ProjectState.IsSubmission)
                 {