[lld-macho] Simplify DeduplicatedCStringSection::finalizeContents. NFC

Tail merge is slow and of low value. With regular string deduplication, we can
just use the return value of StringTableBuilder::add.

There is no noticeable performance increase because without deduplication
`__cstring` is quite small (7.6MiB for chromium_framework).

Reviewed By: #lld-macho, Jez Ng

Differential Revision: https://reviews.llvm.org/D117273