[lld] [llvm] [DTLTO][LLD][ELF] Support bitcode members of thin archives (PR #149425)

[Fangrui Song via llvm-commits]

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

Sorry for the delay. As I mentioned at the Dev Meeting last October, my time for upstream review is limited in my spare time... In addition, click the 'Request Review' circle in the reviewer list :)

https://github.com/llvm/llvm-project/pull/149425