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

[David Blaikie via llvm-commits]

dwblaikie wrote:

I think the right explanation/error message would be "executable does not contain any skeleton units" (technically correct, but probably doesn't mean much to users) or "executable file does not contain any references to dwo files"?

"no dwo files found" sounds more like there weren't any dwo files on disk (and as a user I'd be asking "why didn't you tell me which dwo files you /tried/ to find/couldn't read")

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