[clang] [clang][FMV] Do not omit explicit default target_version attribute. (PR #96628)

[Alexandros Lamprineas via cfe-commits]

labrinea wrote:

@ilinpv, @DanielKristofKiss, the patch https://github.com/llvm/llvm-project/pull/65671/files doesn't seem right. I have basically reverted it with my change, ensuring the original crash you were trying to fix doesn't occur. FYI `MVKind == MultiVersionKind::None` is equivallent to `NewTVA == nullptr` in that context.

https://github.com/llvm/llvm-project/pull/96628