[lld-macho] Deduplicate the `__objc_classrefs` section contents

ld64 breaks down `__objc_classrefs` on a per-word level and deduplicates
them. This greatly reduces the number of bind entries emitted (and
therefore the amount of work `dyld` has to do at runtime). For
chromium_framework, this change to LLD cuts the number of (non-lazy)
binds from 912 to 190, getting us to parity with ld64 in this aspect.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D121053