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

[David Blaikie via llvm-commits]

dwblaikie wrote:

what does binutils dwp do in this case? (not that we should necessarily do the same thing)

(& generally warnings are all lower case, no ending punctuation (no `!` or `.`, etc))

& yeah, test would be good

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