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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194759 91177308-0d34-0410-b5e6-96231b3b80d8