Implement support for pack expansions whose pattern is a non-type

template argument (described by an expression, of course). For
example:

  template<int...> struct int_tuple { };

  template<int ...Values>
  struct square {
    typedef int_tuple<(Values*Values)...> type;
  };

It also lays the foundation for pack expansions in an initializer-list.
  

llvm-svn: 122751