[MC] Plumb unique_ptr<MCELFObjectTargetWriter> through createELFObjectWriter to
ELFObjectWriter's constructor.

Fixes the same ownership issue for ELF that r315245 did for MachO:
ELFObjectWriter takes ownership of its MCELFObjectTargetWriter, so we want to
pass this through to the constructor via a unique_ptr, rather than a raw ptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315254 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed