When we know that we are parsing a class-name, implicitly construct a

TypenameType if getTypeName is looking at a member of an unknown
specialization. This allows us to properly parse class templates that
derived from type that could only otherwise be described by a typename type, 
e.g.,

  template<class T> struct X {};
  template<typename T> struct Y : public X<T>::X { }; 

Fixes PR4381.

llvm-svn: 80123