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

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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315531 91177308-0d34-0410-b5e6-96231b3b80d8