Introduce a new type, PackExpansionType, to capture types that are

pack expansions, e.g. given

  template<typename... Types> struct tuple;

  template<typename... Types>
  struct tuple_of_refs {
    typedef tuple<Types&...> types;
  };

the type of the "types" typedef is a PackExpansionType whose pattern
is Types&. 

This commit introduces support for creating pack expansions for
template type arguments, as above, but not for any other kind of pack
expansion, nor for any form of instantiation.

llvm-svn: 122223