[PATCH v2] LangRef: clarify that va_arg doesn't work on x86_64

[Reid Kleckner]

I think it's possible to modify the example like so, so that it works on
x86_64 with very little modification:

-----------------
; This struct is different for every platform. For most platforms, it is
merely
; an i8*.
%struct.va_list = { i8* }

; For Unix x86_64 platforms, va_list is the following struct:
; %struct.va_list = type { i32, i32, i8*, i8* }

define i32 @test(i32 %X, ...) {
  ; Initialize variable argument processing
  %ap = alloca %struct.va_list
  %ap2 = bitcast %struct.va_list* %ap to i8*
  call void @llvm.va_start(i8* %ap2)

  ; Read a single integer argument
  %tmp = va_arg i8* %ap2, i32

  ; Demonstrate usage of llvm.va_copy and llvm.va_end
  %aq = alloca i8*
  %aq2 = bitcast i8** %aq to i8*
  call void @llvm.va_copy(i8* %aq2, i8* %ap2)
  call void @llvm.va_end(i8* %aq2)

  ; Stop processing of arguments.
  call void @llvm.va_end(i8* %ap2)
  ret i32 %tmp
}

declare void @llvm.va_start(i8*)
declare void @llvm.va_copy(i8*, i8*)
declare void @llvm.va_end(i8*)
-----------------

This generates reasonable code, but va_arg still doesn't handle structs as
mentioned.
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