[PATCH] [lld] [mach-o] Support -l and -syslibroot options

[Tim Northover]

>> I don't think that will work either, at least not universally: it
>> checks that we look for a file, but not what we do with it once found.
>> There may well be a use for such an option, but tests like this are
>> needed too.
> Can you give me an example where “what we do with it” matters?

All of them. We won't be calling MachOLinkingContext::fileExists for
its side-effects, so we want to know that its return value is treated
correctly (at every callsite).

For this particular situation we want to test that if the
library/object actually exists, it gets added to the InputGraph and
actually used during the link phase.

Cheers.

Tim.