[llvm-branch-commits] [NFC][ELF] Move mergeRels/partitionRels into finalizeContents (PR #171203)

[Jessica Clarke via llvm-branch-commits]

https://github.com/jrtc27 created https://github.com/llvm/llvm-project/pull/171203

Other than the ordering requirements that remain between sections, this
abstracts the details of how these sections are implemented.

Note that isNeeded already checks relocsVec for both section types, so
finalizeSynthetic can call it before mergeRels just fine.