[LLD][COFF] Factor out addObjectFile (NFC) (#206811)
Avoid parsing the input COFF file twice: first, implicitly, in
findBitcodeInObject and later when creating the ObjFile instance. Create
the COFFObjectFile earlier and use it for both the findBitcodeInObject
call and ObjFile creation. It's also preparation for handling hybrid
ARM64X object files.
