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

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

Please make sure to verify this works in C23 mode as well. e.g.,
```
void func(...) {}

void foo() {
  va_list list;
  va_start(list);
  ...
  va_end(list);
}
```
I suspect it will Just Work™ but an explicit test for that scenario would be useful IMO.

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