[llvm] Improve error output for duplicate symbol for `llvm-link` and `lto` (PR #118712)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

> I'm working on fixing the test failure, but meanwhile would like to get guidance on whether it is possible to get the paths of both modules that caused a symbol collision.

Unlikely, because these lower level LLVM mechanisms don't keep track of source modules for already merged symbols. They are really only meant for LLVM testing though. Better to use a linker for LTO linking anything real, which will give better error messages.

https://github.com/llvm/llvm-project/pull/118712