[lld-macho] Only fold private-label aliases that do not have flags

This will enable us to re-land {D139069}.

The issue with the original diff was that we were folding all
private-label symbols. We were not merging the symbol flags during this
folding; instead we just made all references to the folded symbol point
to its aliasee. This caused some flags to be incorrectly discarded. This
surfaced as code that was incorrectly stripped due to LLD dropping the
`.no_dead_strip` flag.

This diff fixes things by only folding flag-less private-label aliases.
Most (maybe all) of the `ltmp<N>` symbols that are generated by the MC
aarch64 backend are flag-less, so this conservative folding behavior
does the job.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D140606