Add proxies classes for raw_ostream and raw_pwrite_stream called
raw_ostream_proxy and raw_pwrite_stream_proxy. Add adaptor classes,
raw_ostream_proxy_adaptor<> and raw_pwrite_stream_proxy_adaptor<>,
to allow subclasses to use a different parent class than raw_ostream
The adaptors are used by a future patch to help a subclass of
llvm::vfs::OutputFile, an abstract subclass of raw_pwrite_stream, to
proxy a raw_fd_ostream.
Patched by dexonsmith.