[llvm] [llvm][object] Expose `writeArchiveToStream` (PR #98254)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I am also fine with exposing the API. If this helps some projects you are working on, it's best to mention the use case, though.
An in-tree use case is particular useful as otherwise there is always a risk that future contributors might notice this export unneeded and refactor code to internalize it again.

https://github.com/llvm/llvm-project/pull/98254