[llvm] [Object][Archive] Recompute headers and symbol map when switching from COFF to GNU64 (PR #160606)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Can we represent everything we need in gnu format archives?  Like, if we don't emit the symbol map, will that cause problems for a COFF linker?

https://github.com/llvm/llvm-project/pull/160606