[llvm] [Linker] Do not keep a private member of a non-prevailing comdat group (PR #69143)

[Reid Kleckner via llvm-commits]

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

The premerge test failure looks unrelated. I think this is good, with a small modification.

https://github.com/llvm/llvm-project/pull/69143