[llvm] [llvm-ar] Use COFF archive format for COFF targets. (PR #82642)

[Martin Storsjö via llvm-commits]

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

LGTM, thanks! This looks like a very straightforward refactoring, with a minimal behaviour change on top.

(In principle, this could maybe have been split into a pure NFC refactoring, then a separate step that only adds the `COFF` cases on top, but I don't think it's worth the extra effort here, as it's trivial enough anyway, and stacked PRs are annoying.)

https://github.com/llvm/llvm-project/pull/82642