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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Would https://llvm.org/docs/TestingGuide.html#elaborated-tests help make the tests cleaner/easier to maintain? (or is there enough post-processing of the files, needs other tools, etc?)

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