Implement the notions of the "current instantiation" and "unknown

Implement the notions of the "current instantiation" and "unknown
specialization" within a C++ template, and permit name lookup into the
current instantiation. For example, given:

  template<typename T, typename U>
  struct X {
    typedef T type;

    X* x1;  // current instantiation
    X<T, U> *x2; // current instantiation
    X<U, T> *x3; // not current instantiation
    ::X<type, U> *x4; // current instantiation
    X<typename X<type, U>::type, U>: *x5; // current instantiation
  };

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71471 91177308-0d34-0410-b5e6-96231b3b80d8