[llvm] [LTO][NFC] Switch LTO API from output parameter to return value (PR #151272)

[Peter Collingbourne via llvm-commits]

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

So after this change, in #150690 we would have addRegularLTO taking an additional argument with all of the resolutions, something like `ArrayRef<Resolution> FullRes`? Seems okay to me.

https://github.com/llvm/llvm-project/pull/151272