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

[Jinjie Huang via llvm-commits]

Labman-001 wrote:

Thank you for the review comments.

> Also maybe print out which file was not found?

Since the scenario here targets the branch where the dwo path list is empty, there might not be a concept of "which" here. In regular scenarios, if a specific dwo file is missing, the 'write' function will report an error indicating that the corresponding file cannot be found.

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

In my Debian 10 system(Binutils dwp 2.31.1),  it will just trigger a core dump in this case, while llvm-dwp just quits silently：
```
> dwp -e xxx.out -o xxx.dwp
Segmentation fault (core dumped)

>llvm-dwp xxx.out -o xxx.dwp
```
So it may be helpful to give a warning here?

>& yeah, test would be good
Yes, I will add a  targeted test.

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