Specialize the constructors for DIRef<DIScope> and DIRef<DIType> to make sure
the Value is indeed a scope ref and a type ref.
Use DIScopeRef for DIScope::getContext and DIType::getContext and use DITypeRef
for getContainingType and getClassType.
DIScope::generateRef now returns a DIScopeRef instead of a "Value *" for
readability and type safety.
llvm-svn: 190418