[lld] [lld][test] Use LLVM helpers for input file discovery (PR #220404)

[Michał Górny via llvm-commits]

mgorny wrote:

The error I was getting was:

```
CMake Error: File /var/tmp/portage/llvm-core/lld-24.0.0.9999/work/llvm/unittests/unittest.cfg.in does not exist.
CMake Error at /usr/lib/llvm/24/lib64/cmake/llvm/AddLLVM.cmake:2099 (configure_file):
  configure_file Problem configuring file
Call Stack (most recent call first):
  unittests/AsLibELF/CMakeLists.txt:5 (add_unittest_with_input_files)


CMake Error at unittests/AsLibELF/CMakeLists.txt:11 (target_link_libraries):
  Cannot specify link libraries for target "LLDAsLibELFTests" which is not
  built by this project.


-- Configuring incomplete, errors occurred!
```

I haven't investigated beyond that, given the comment here. It might just be a matter of making the sparse checkout less sparse.

https://github.com/llvm/llvm-project/pull/220404