Add raw_ostream wrapper to the Stream class

Summary:
This wrapper will allow us in the future to reuse LLVM methods from within the
Stream class.

Currently no test as this is intended to be an internal class that shouldn't have any
NFC. The test for this change will be the follow up patch that migrates LLDB's
LEB128 implementation to the one from LLVM.

This change also adds custom move/assignment methods to Stream, as LLVM
raw_ostream doesn't support these. As our internal stream has anyway no state,
we can just keep the same stream object around.

Reviewers: davide, labath

Reviewed By: labath

Subscribers: xiaobai, labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D50161

llvm-svn: 338901