[PATCH] Fix "#pragma clang __debug overflow_stack" for MSVC builds

[Richard Smith]

================
Comment at: lib/Lex/Pragma.cpp:928-932
@@ -927,6 +927,7 @@
 #endif
   static void DebugOverflowStack() {
+    void* StackUser = &StackUser;
     void (*volatile Self)() = DebugOverflowStack;
     Self();
   }
 #ifdef _MSC_VER
----------------
rsmith wrote:
> A smart compiler could still optimize out the stack usage here. How about using something that keeps all the stack memory live, such as:
> 
>   volatile void *Head;
>   static void DebugOverflowStack(void *Curr = 0) {
>     Head = Curr;
>     DebugOverflowStack(&Curr);
>   }
> 
> Hopefully MSVC doesn't optimize out stores to volatile globals...
Sorry, should be

  void *volatile Head;

http://reviews.llvm.org/D5109