[llvm] [MC] Move MCFragment::Atom to MCSection::Atoms (PR #95341)

[via llvm-commits]

https://github.com/aengelke approved this pull request.

LGTM. Minor thing: We could resize the vector once in `MCMachOStreamer::finishImpl` with LayoutOrder+1 of the last fragment. This would avoid a resize call for every fragment (=> log n actual resizes) and getAtom/setAtom would not need index checks.

https://github.com/llvm/llvm-project/pull/95341