[lld-macho] Support --thinlto-jobs

The test is loosely based off LLD-ELF's `thinlto.ll`. However, I
found that test questionable because the the -save_temps behavior it
checks for is identical regardless of whether we are running in single-
or multi-threaded mode. I tried writing a test based on `--time-trace`
but couldn't get it to run deterministically... so I've opted to just
skip checking that behavior for now.

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D99356