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

[Fangrui Song via llvm-commits]

MaskRay wrote:

`LayoutOrder` should probably be replaced with `Ordinal`. Sent #97474 to clarify that `LayoutOrder` is Mach-O specific.
The overall complexity decreases, so LGTM :) 

> Remove some maps. Unfortunately, it didn't result in a measurable performance improvements, but at least some parts of this are a nice to have regardless.

Understood. While a relocatable file might have many sections due to -ffunction-sections -fdata-sections, they are unlikely bottlenecks.

https://github.com/llvm/llvm-project/pull/97421