Docs: update va_arg example with valid x86_64 va_list type.

The given example was overflowing its alloca and segfaulting if actually run on
x86, so it's a good idea to provide something that works there too.

Patch by Ramkumar Ramachandra.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221077 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -6884,14 +6884,21 @@ variable argument handling intrinsic functions are used.
 
 .. code-block:: llvm
 
+    ; This struct is different for every platform. For most platforms,
+    ; it is merely an i8*.
+    %struct.va_list = type { 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 i8*
-      %ap2 = bitcast i8** %ap to i8*
+      %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** %ap, i32
+      %tmp = va_arg i8* %ap2, i32
 
       ; Demonstrate usage of llvm.va_copy and llvm.va_end
       %aq = alloca i8*