[MC] Plumb unique_ptr<MCWinCOFFObjectTargetWriter> through

createWinCOFFObjectWriter to WinCOFFObjectWriter's constructor.

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

llvm-svn: 315257