Function emit_va_arg 

pub(crate) fn emit_va_arg<'ll, 'tcx>(
    bx: &mut GenericBuilder<'_, 'll, FullCx<'ll, 'tcx>>,
    addr: OperandRef<'tcx, Value<'ll>>,
    layout: TyAndLayout<'tcx>,
) -> Value<'ll>

Determine the va_arg implementation to use. The LLVM va_arg instruction is lacking in some instances, so we should only use it as a fallback.

https://llvm.org/docs/LangRef.html#va-arg-instruction