[lld-macho] Deduplicate CFStrings

`__cfstring` is a special literal section, so instead of breaking it up
at symbol boundaries, we break it up at fixed-width boundaries (since
each literal is the same size). Symbols can only occur at one of those
boundaries, so this is strictly more powerful than
`.subsections_via_symbols`.

With that in place, we then run the section through ICF.

This change is about perf-neutral when linking chromium_framework.

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D105045