Previously Ostream was a class that wrapped a raw_ostream pointer,
structured that way in case we wanted to wrap an alternate stream
type.

Also, Ostream used to include a Cfg pointer, but that had to go away
when the Ostream became associated with the GlobalContext which
persists beyond the Cfg lifetime, so the Cfg pointer was removed
leaving only the raw_ostream.

Since llvm::raw_ostream is supposed to be very lightweight, we can
just give up the abstraction and equate it to Ice::Ostream.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/413393005