[compiler-rt] Fix broken unittest using alloca on MSVC.

[etienneb]

Summary

The alloca header is not present on windows.

This test was committed recently:

http://reviews.llvm.org/D21509
http://reviews.llvm.org/rL273889

[rnk]

lgtm

test/asan/TestCases/alloca_constant_size.cc

 // Regression test for https://github.com/google/sanitizers/issues/691
 
 // RUN: %clangxx_asan -O0 %s -o %t -fstack-protector
 // RUN: %run %t 1 2>&1 | FileCheck %s
 // RUN: %run %t 2 2>&1 | FileCheck %s
 
-#include <alloca.h>
 #include <stdio.h>
 #include <string.h>
 
+// MSVC provides _alloca instead of alloca.
+#if defined(_MSC_VER) && !defined(alloca)
+# define alloca _alloca
+#else
+#include <alloca.h>
+#endif
+
+
 void f1_alloca() {
   char *dynamic_buffer = (char *)alloca(200);
   fprintf(stderr, "dynamic_buffer = %p\n", dynamic_buffer);
   memset(dynamic_buffer, 'y', 200);
   return;
 }
 
 static const int kDynamicArraySize = 200;