[lld-macho] Have toString() emit full path to archive files

It doesn't make sense to take just the base filename for archives when we emit
the full path for object files. (LLD-ELF emits the full path too.)

This will also make it easier to write a proper test for {D100147}.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D100357