[lld-macho] Ignore LLVM segments to prevent duplicate syms

There was an instance of a third-party archive containing multiple
_llvm symbols from different files that clashed with each other
producing duplicate symbols. Symbols under the LLVM segment
don't seem to be producing any meaningful value, so just ignore them.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D108016