Fix a crashing but trying to print a TemplateTemplateParmDecl

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108007 91177308-0d34-0410-b5e6-96231b3b80d8