Replace the representation of template template argument pack

Replace the representation of template template argument pack
expansions with something that is easier to use correctly: a new
template argment kind, rather than a bit on an existing kind. Update
all of the switch statements that deal with template arguments, fixing
a few latent bugs in the process. I"m happy with this representation,
now.

And, oh look! Template instantiation and deduction work for template
template argument pack expansions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122896 91177308-0d34-0410-b5e6-96231b3b80d8