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

A va_start intrinsic lowers to something derived from the variadic
parameter to the function. If there is no such parameter, it can't lower
meaningfully. Clang sema rejects the same with `error: 'va_start' used
in function with fixed args`.

Moves the existing lint warning into a verifier error. Updates the one
lit test that had a va_start in a non-variadic function.