Implicitly expand argument packs when performing template argument

deduction. Unify all of the looping over template arguments for
deduction purposes into a single place, where argument pack expansion
occurs; this is also the hook for deducing from pack expansions, which
itself is not yet implemented.

For now, at least we can handle a basic "count" metafunction written
with variadics. See the new test for the formulation that works.

llvm-svn: 122418