[lld/mac] Implement basic typo correction for flags

[thakis]

Summary

Also use "unknown flag 'flag'" instead of "unknown flag: flag" for
consistency with the other ports.

[hans]

lld/MachO/DriverUtils.cpp

78–85

i probably would have kept opt::Arg here, but i guess it's a personal preference

lld/test/MachO/driver.test

2

nit: blank line after run line is common (or am i just imagining this?)

[thakis]

Thanks! Landing.