[dsymutil] Fix conversion between unique_ptr and Expected

Reproducer.cpp:70:12: error: could not convert ‘Repro’ from
‘std::unique_ptr<llvm::dsymutil::ReproducerGenerate,
std::default_delete<llvm::dsymutil::ReproducerGenerate> >’ to
‘llvm::Expected<std::unique_ptr<llvm::dsymutil::Reproducer> >’