[lld-macho] Construct CFString literals by copying the ConcatInputSection

... instead of constructing a new one each time. This allows us
to take advantage of {D105305}.

I didn't see a substantial difference when linking chromium_framework,
but this paves the way for reusing similar logic for splitting compact
unwind entries into sections. There are a lot more of those, so the
performance impact is significant.

Differential Revision: https://reviews.llvm.org/D109895