Add a simple type-safe bit-mangling pointer union class. This allows

you to do things like:

  ///    PointerUnion<int*, float*> P;
  ///    P = (int*)0;
  ///    printf("%d %d", P.is<int*>(), P.is<float*>()); // prints "1 0"
  ///    X = P.get<int*>();     // ok.
  ///    Y = P.get<float*>();   // runtime assertion failure.
  ///    Z = P.get<double*>();  // does not compile.
  ///    P = (float*)0;
  ///    Y = P.get<float*>();   // ok.
  ///    X = P.get<int*>();     // runtime assertion failure.

llvm-svn: 67987