[lld] [lld-macho] Implement symbol string deduplication (PR #123874)

[Vincent Lee via llvm-commits]

https://github.com/thevinster approved this pull request.

Chatted offline. The two flags are fine. Apple's `-no_deduplicate` seems to be orthogonal to symbol string deduplication, and combining it under one flag could create confusion. Two flags also provides more flexibility. 

https://github.com/llvm/llvm-project/pull/123874