Don't assert when trying to re-expand a template. See case #542749

In some very specific situation where there's classes that inherit
from a generic base class like:

BaseClass<T> : MonoBehaviour where T : BaseClass<T>
AClass : BaseClass<AClass>

the compiler tries to expand the templates twice. *boggle*
Merge pull request #70 from Unity-Technologies/unity-staging

Don't assert when trying to re-expand a template. See case #542749