[lld] [LLD][COFF] Use lazy object mechanism instead of relying on the archive map for thin archives on ARM64EC (PR #194349)

[Martin Storsjö via llvm-commits]

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

I think this looks good, thanks!

Can you describe what the "lazy object mechanism" practically means here? Previously when loading a thin archive, we'd just read the archive itself, and only open the individual files on disk if they are needed to fulfill some missing symbol, right? (Unless wholearchive is specified of course.) But now with lazy loading, we do actually open each individual object file and do some initial parsing of it, but not actually pull in its contents?

In which existing cases do we use the same "lazy object" mechanism? I tried to look around, and it seems it is used for the `-start-lib` option only?

https://github.com/llvm/llvm-project/pull/194349