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

[Jinjie Huang via llvm-commits]

Labman-001 wrote:

The message has been changed to **'executable file does not contain any references to dwo files'**. 
But In fact, there are two situations where `DWOFilenames.empty()` is true: one is when there are no references to dwo files in the skeleton, and the other is when no dwo files are given in the input.
Should we consider changing the message to "no dwo files are provided or executable file does not contain any references to dwo files"?

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