[lld/mac] Fix --start-lib/--end-lib with split thinlto inputs

Fixes #59162. The test has a comment explaining what's going on.
See also Symbol::extract() in lld/ELF/Symbols.cpp.

The included test sadly also passes if I pass just bd448f01,
while doing that isn't enough to make my bigger repro case work
(if I port just that, something else asserts later on, but with
this fix here everything's fine in my bigger repro).

Differential Revision: https://reviews.llvm.org/D139199