Implement support for non-type template parameter packs whose type is

Implement support for non-type template parameter packs whose type is
a pack expansion, e.g., the parameter pack Values in:

  template<typename ...Types>
  struct Outer {
    template<Types ...Values>
    struct Inner;
  };

This new implementation approach introduces the notion of an
"expanded" non-type template parameter pack, for which we have already
expanded the types of the parameter pack (to, say, "int*, float*",
for Outer<int*, float*>) but have not yet expanded the values. Aside
from creating these expanded non-type template parameter packs, this
patch updates template argument checking and non-type template
parameter pack instantiation to make use of the appropriate types in
the parameter pack.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123845 91177308-0d34-0410-b5e6-96231b3b80d8