[lld-macho] Allow dead_strip to work with exported private extern symbols

It seems like we are overly asserting when running `-dead_strip` with
exported symbols. ld64 treats exported private extern symbols as a liveness
root. Loosen the assert to match ld64's behavior.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D124143