[MC] Plumb unique_ptr<MCMachObjectTargetWriter> through createMachObjectWriter

[MC] Plumb unique_ptr<MCMachObjectTargetWriter> through createMachObjectWriter
to MCObjectWriter's constructor.

MCObjectWriter takes ownership of its MCMachObjectTargetWriter argument -- this
patch plumbs that ownership relationship through the constructor (which
previously took raw MCMachObjectTargetWriter*) and the createMachObjectWriter
function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315245 91177308-0d34-0410-b5e6-96231b3b80d8