As discussed in https://github.com/llvm/llvm-project/issues/53475 this patch allows using LLD-as-a-lib. It also lets clients link-in only the drivers that they want (see tests).
This also adds the unittests infra as in the other LLVM projects. Among the test coverage, I've added the original issue from @krzysz00, see this.
Important note: this doesn't allow (yet) linking in parallel. This will come a bit later, in subsequent patches, for COFF at last.