Implement transformation of template names within the generic tree

transform, then use the result for template instantiation. The generic
transformation fixes a few issues:

  - It copes better with template template parameters and member
  templates (when they're implemented). 
  - The logic used to replace template template parameters with their
  arguments is now centralized in TransformDecl, so that it will apply
  for other declaration-instantiation steps.
  - The error-recovery strategy is normalized now, so that any error
  results in a NULL TemplateName.

llvm-svn: 78292