[MC][ELF] Eliminate some hash maps from ELFObjectWriter (#97421)

Remove some maps. Mostly cleanup, only a slight performance win.

- Replace SectionIndexMap with layout order: The section layout order is
only used in MachO, so we can repurpose the field as section table
index.
- Store section offsets in MCSectionELF: No need for a map, and
especially not a std::map. Direct access to the underlying (and easily
modifyable) data structure is always faster.
- Improve storage of groups: There's no point in having a DenseMap, the
number of sections and groups are reasonably small to use vectors.

0 files changed, 0 insertions, 0 deletions