[mach-o] support N_NO_DEAD_STRIP nlist.desc bit

Mach-O symbols can have an attribute on them means their content should never be
dead code stripped.  This translates to deadStrip() == deadStripNever.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@216234 91177308-0d34-0410-b5e6-96231b3b80d8