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

[Michał Górny via llvm-commits]

mgorny wrote:

This also seems to introduce a source dependency between LLD and LLVM unit tests. Given https://github.com/llvm/llvm-project/pull/220404#issuecomment-5838280863, is this going to be reverted before I waste my time trying to getting it to work with standalone builds?

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