[llvm] [llvm-ar][Object] Use K_GNU instead of K_COFF for archives with no symtab. (PR #82898)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I was going to ask what MSVC does... but apparently, it specifically forbids creating empty archives (LNK1160), so there's no equivalent.

If someone creates an archive, then add something to it/removes something from it, does llvm-ar/llvm-lib rerun the heuristic for choosing an archive format?  If it does, this is probably fine.

https://github.com/llvm/llvm-project/pull/82898