Initial infrastructure for class template partial specialization. Here

Initial infrastructure for class template partial specialization. Here
we have the basics of declaring and storing class template partial
specializations, matching class template partial specializations at
instantiation time via (limited) template argument deduction, and
using the class template partial specialization's pattern for
instantiation.

This patch is enough to make a simple is_pointer type trait work, but
not much else.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72662 91177308-0d34-0410-b5e6-96231b3b80d8