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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122223 91177308-0d34-0410-b5e6-96231b3b80d8