[lld] [ELF] Set vna_flags to VER_FLG_WEAK if all references are weak (PR #176673)

[Peter Smith via llvm-commits]

smithp35 wrote:

> GNU ld doesn't set `vna_flags` (for version need) to the best of my knowledge. It sets `vd_flags` (for version definition) when the version node has no associated definitions. This behavior, probably inherited from Solaris doesn't seem useful, and is completed ignored by Linux and FreeBSD's rtld implementations.
> 
The bit that I can see the `VER_FLG_WEAK` getting set is in `_bfd_elf_link_find_version_dependencies`
  a->vna_flags = h->verinfo.verdef->vd_flags;

However I think this is still likely a holdover from Solaris.

https://github.com/llvm/llvm-project/pull/176673