Implicitly expand argument packs when performing template argument

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122418 91177308-0d34-0410-b5e6-96231b3b80d8