[llvm] [LTO][Legacy] Add new C APIs to query undefined symbols in assembly (PR #145413)

[Teresa Johnson via llvm-commits]

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

This seems ok to make legacy thin LTO equivalent to legacy full LTO behavior. I don't know that the new LTO api handles this case for full or thin LTO? Does that issue remain unresolved for lld/gold?

https://github.com/llvm/llvm-project/pull/145413