We're seeing a performance issue in Razor projects in cohosting scenarios. The first run of the Razor generator is relatively slow, and so ideally we wouldn't have that happen more than once. However, since multiple features might request compilations for a Razor project during startup in parallel, it's possible that all of those parallel invocations might initialize the generator multiple times if they happen to be different Solution instances. This change tries to ensure that when we're creating a GeneratorDriver for a project when we don't already have one, we'll do that computation just once, and try to share that across the forks. This ensures we do the expensive initialization only once.