Implement transformation of template names within the generic tree

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78292 91177308-0d34-0410-b5e6-96231b3b80d8