[MC] Plumb unique_ptr<MCWasmObjectTargetWriter> through createWasmObjectWriter

to WasmObjectWriter's constructor.

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

llvm-svn: 315260