[llvm] [yaml2obj][XOFF] Update yaml2obj for XCOFF to create valid XCOFF files in more cases. (PR #77620)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

> > > I haven't attempted to trace things fully, but the `AuxHeaderSize` field is now a `std::optional` in this patch. Is that not something that is exposed to the user?
> 
> I thought I had a reason to make AuxHeaderSize optional , but I it looks like that specific change isn't necessary, at least for now. I'm not sure what you mean by exposing an optional field to the user. In any case, the XCOFF auxiliary header support needs some additional changes (in a future PR--the auxiliary fields can be read by yaml2obj, but are not currently printed by obj2yaml.

I don't know why, but I had in my head that you'd changed code that might mean that people could omit a field that they couldn't before, in which case that should be tested. Looking again, I don't think there is, sorry for the fuss and LGTM.

https://github.com/llvm/llvm-project/pull/77620