[llvm] [llvm] Fix GNU/Hurd build (PR #184470)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay requested changes to this pull request.

Should do something like
```
  +  SmallString<128> Buf;
  +  if (!sys::fs::readlink(Path, Buf)) {
  +    std::string s(Buf.str());
  +    LibPathCache->insert_link(Path, s);
```

https://github.com/llvm/llvm-project/pull/184470