[llvm] [Verifier] Reject va_start in non-variadic function (PR #88809)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Do we want AutoUpgrade for this, or should we assume this doesn't show up in any existing bitcode files anyone cares about?

Otherwise looks fine.

https://github.com/llvm/llvm-project/pull/88809