GoToDefinition is not available in [metadata] files

[Arnavion]

• VSCode Version: 1.5.2
• OS Version: Windows 7 x64

Steps to Reproduce:

1. Have a C# project with the content:

   class Foo
   {
       System.Collections.Generic.List<int> bar;
   }

2. Invoke F12 GoToDefinition on List to be taken to the metadata file for List1 (public class List : IList...`

3. In this file, GoToDefinition is not an option on IList<T> and pressing F12 on it does nothing.

Other language service things like tooltips are also not available.

[chrmarti]

Tracking as dotnet/vscode-csharp#771