[MC] Have MCObjectStreamer take its MCAsmBackend argument via unique_ptr.

MCObjectStreamer owns its MCAsmBackend -- this fixes the types to reflect that,
and allows us to remove another instance of MCObjectStreamer's weird "holding
ownership via someone else's reference" trick.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315410 91177308-0d34-0410-b5e6-96231b3b80d8
39 files changed