This changes the CompilerInstance::createOutputFile function to return
a std::unique_ptr<llvm::raw_ostream>, rather than an llvm::raw_ostream
implicitly owned by the CompilerInstance. This in most cases required that
I move ownership of the output stream to the relevant ASTConsumer.
The motivation for this change is to allow BackendConsumer to be a client
of interfaces such as D20268 which take ownership of the output stream.
I think return nullptr; would be clearer here.