Include raw_ostream.h.

Including only Debug.h did not cause a compilation error, but you couldn't
do anything (like writing something with <<) to raw_ostreams returned by
llvm::dbgs() or llvm::errs() without including raw_ostream.h. So including
it from Debug.h should make sense.

Differential Revision: http://llvm-reviews.chandlerc.com/D2183