[LLD] Allow usage of LLD as a library

As discussed in https://github.com/llvm/llvm-project/issues/53475 this patch allows using LLD-as-a-lib. It also lets clients link only the drivers that they want (see unit tests).

This also adds the unit test infra as in the other LLVM projects. Among the test coverage, I've added the original issue from @krzysz00, see: https://github.com/ROCmSoftwarePlatform/D108850-lld-bug-reproduction

Important note: this doesn't allow (yet) linking in parallel. This will come a bit later, in subsequent patches, for COFF at last.

Differential revision: https://reviews.llvm.org/D119049