[lld-macho] Fold cfstrings with --deduplicate-literals

Similar to cstrings ld64 always deduplicates cfstrings. This was already
being done when enabling ICF, but for debug builds you may want to flip
this on if you cannot eliminate your instances of this, so this change
makes --deduplicate-literals also apply to cfstrings.

Differential Revision: https://reviews.llvm.org/D130134