Fix build failure on windows (NFC)

C:\buildbot\mlir-x64-windows-ninja\llvm-project\llvm\lib\Object\MachOUniversalWriter.cpp(352) : error C2220: the following warning is treated as an error
C:\buildbot\mlir-x64-windows-ninja\llvm-project\llvm\lib\Object\MachOUniversalWriter.cpp(352) : warning C4715: 'llvm::object::writeUniversalBinaryToStream': not all control paths return a value