PNaCl: Fix handling of va_arg on structs in simplification passes

lib/Transforms/NaCl/ExpandVarArgs.cpp

 //===- ExpandVarArgs.cpp - Expand out variable argument function calls-----===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This pass expands out all use of variable argument functions.
 //
 // This pass replaces a varargs function call with a function call in
 // which a pointer to the variable arguments is passed explicitly.
 // The callee explicitly allocates space for the variable arguments on
 // the stack using "alloca".
 //
 // Alignment:
 //
 // This pass does not add any alignment padding between the arguments
-// that are copied onto the stack.  We assume that the only argument
-// types that need to be handled are 32-bit and 64-bit -- i32, i64,
-// pointers and double:
+// that are copied onto the stack.  This means that if the argument
+// list contains a mixture of, say, 1-byte and 4-byte values, the code
+// generated by this pass might be inefficient due to the memory
+// accesses being unaligned.
 //
-//  * We won't see i1, i8, i16 and float as varargs arguments because
-//    the C standard requires the compiler to promote these to the
-//    types "int" and "double".
-//
-//  * We won't see va_arg instructions of struct type because Clang
-//    does not yet support them in PNaCl mode.  See
-//    https://code.google.com/p/nativeclient/issues/detail?id=2381
-//
-// If such arguments do appear in the input, this pass will generate
-// correct, working code, but this code might be inefficient due to
-// using unaligned memory accesses.
+// This should only be an issue when passing structs as varargs
+// arguments.  We won't see i1, i8, i16 and float as varargs arguments
+// because the C standard requires the compiler to promote these to
+// the types "int" and "double".
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
@@ skipping 284 matching lines @@
       ExpandVarArgFunc(Func);
     }
   }
 
   return Changed;
 }
 
 ModulePass *llvm::createExpandVarArgsPass() {
   return new ExpandVarArgs();
 }