specialization of class templates, e.g.,

  template<typename T> class X;

  template<> class X<int> { /* blah */ };

Each specialization is a different *Decl node (naturally), and can
have different members. We keep track of forward declarations and
definitions as for other class/struct/union types.

This is only the basic framework: we still have to deal with checking
the template headers properly, improving recovery when there are
failures, handling nested name specifiers, etc.

llvm-svn: 64848