Start implementing support for substitution into pack expansions that

Start implementing support for substitution into pack expansions that
involve template parameter packs at multiple template levels that
occur within the signatures members of class templates (and partial
specializations thereof). This is a work-in-progress that is deficient
in several ways, notably:
  - It only works for template type parameter packs, but we need to
  also support non-type template parameter packs and template template
  parameter packs.
  - It doesn't keep track of the lengths of the substituted argument
  packs in the expansion, so it can't properly diagnose length
  mismatches.

However, this is a concrete step in the right direction.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123425 91177308-0d34-0410-b5e6-96231b3b80d8