[MC] Allow MCObjectWriter's output stream to be swapped out

There are occasions where it is useful to consider the entirety of the
contents of a section.  For example, compressed debug info needs the
entire section available before it can compress it and write it out.
The compressed debug info scenario was previously implemented by
mirroring the implementation of writeSectionData in the ELFObjectWriter.

Instead, allow the output stream to be swapped on demand.  This lets
callers redirect the output stream to a more convenient location before
it hits the object file.

No functionality change is intended.

Differential Revision: http://reviews.llvm.org/D12509

llvm-svn: 246554