[Driver] Place -mharden-sls= in m_Group

Fix https://github.com/llvm/llvm-project/issues/63237

With only a link action, we claim all CompileOnly_Group options (including -f*,
-m*, -i*, etc). -mharden-sls= is not in a Group and therefore not claimed,
leading to a spurious -Wunused-command-line-argument warning, and after
5548843d, an error.