[lld] [llvm] [lld][COFF] Remove duplicate strtab entries (PR #141197)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I think this looks reasonable, thanks! I'd prefer to have an ack from @MaskRay for the additions to the shared MC level StringTableBuilder class though, but if he doesn't follow up in a few days I guess I can approve this as well.

https://github.com/llvm/llvm-project/pull/141197