[MC] Plumb unique_ptr<MCWinCOFFObjectTargetWriter> through

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315257 91177308-0d34-0410-b5e6-96231b3b80d8