[mach-o] Add support for -S option

The darwin linker does not process dwarf debug info.  Instead it produces a
"debug map" in the output file which points back to the original .o files for
anything that wants debug info (e.g. debugger).

The -S option means "don't add a debug map".  lld for mach-o currently does
not generate the debug map, so there is nothing to do when this option is used.
But we need to process the option to get existing projects building.
[ELF] Remove llvm::Twine usage