Fix a crashing but trying to print a TemplateTemplateParmDecl

for code like this:
   template<template<typename T> class U> class V {};

The problem is that the DeclPrinter assumed all TemplateDecls
have a getTemplatedClass(), but template template params don't
(so we got a NULL dereference).  The solution is to detect if
we're a template template param, and construct the template
class name ('class U') specially in this case.

OKed by dgregor and chandlerc

llvm-svn: 108007