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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@80123 91177308-0d34-0410-b5e6-96231b3b80d8