Fix a behavioral change accidentally introduced by the move to the LLVM commandline system: `-output-(bc|ll|s)` used to disable generation of an object file unless explicitly requested with `-output-o`. Now they do so again.

#ifndef LDC_GEN_TOOBJ_H
#define LDC_GEN_TOOBJ_H

void writeModule(llvm::Module* m, std::string filename);

#endif