Implement basic support for template instantiation of pack expansions

whose patterns are template arguments. We can now instantiate, e.g.,

  typedef tuple<pair<OuterTypes, InnerTypes>...> type;

where OuterTypes and InnerTypes are template type parameter packs.

There is a horrible inefficiency in
TemplateArgumentLoc::getPackExpansionPattern(), where we need to
create copies of TypeLoc data because our interfaces traffic in
TypeSourceInfo pointers where they should traffic in TypeLocs
instead. I've isolated in efficiency in this one routine; once we
refactor our interfaces to traffic in TypeLocs, we can eliminate it.

llvm-svn: 122278