[llvm] Give a warning when no dwo files are provided (PR #94336)

[David Blaikie via llvm-commits]

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

This is OK
Though, if you can - https://llvm.org/docs/TestingGuide.html#elaborated-tests - use this so you can include the source code in the test, and make it easy to regenerate, if needed?

https://github.com/llvm/llvm-project/pull/94336