[llvm-libtool-darwin] Refactor ArchiveWriter

Refactoring function `writeArchive` in ArchiveWriter. Added a new
function `writeArchiveBuffer` that returns the archive in a memory
buffer instead of writing it out to the disk. This refactor is necessary
so as to allow `llvm-libtool-darwin` to write universal files containing
archives.

Reviewed by jhenderson, MaskRay, smeenai

Differential Revision: https://reviews.llvm.org/D84858