[MC] Thread unique_ptr<MCObjectWriter> through the create.*ObjectWriter
functions.

This makes the ownership of the resulting MCObjectWriter clear, and allows us
to remove one instance of MCObjectStreamer's bizarre "holding ownership via
someone else's reference" trick.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315327 91177308-0d34-0410-b5e6-96231b3b80d8
55 files changed