[ELF] --orphan-handling=: don't warn/error for input SHT_REL[A] retained by --emit-relocs

They are purposefully skipped by input section descriptions (rL295324).
Similarly, --orphan-handling= should not warn/error for them.
This behavior matches GNU ld.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D75151