Add semantic analysis for the creation of and an AST representation

Add semantic analysis for the creation of and an AST representation
for template template argument pack expansions. This allows fun such
as:

  template<template<class> class ...> struct apply_impl { /*...*/ };
  template<template<class> class ...Metafunctions> struct apply {
    typedef typename apply_impl<Metafunctions...>::type type;
  };

However, neither template argument deduction nor template
instantiation is implemented for template template argument packs, so
this functionality isn't useful yet.

I'll probably replace the encoding of template template
argument pack expansions in TemplateArgument so that it's harder to
accidentally forget about the expansion. However, this is a step in
the right general direction.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122890 91177308-0d34-0410-b5e6-96231b3b80d8