[asan] make __asan::Deallocate immune to racy double-free (issue #57)

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@154097 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/asan/asan_internal.h b/lib/asan/asan_internal.h
index 39d3adf..3a25cf2 100644
--- a/lib/asan/asan_internal.h
+++ b/lib/asan/asan_internal.h
@@ -187,6 +187,7 @@
 int GetPid();
 uintptr_t GetThreadSelf();
 int AtomicInc(int *a);
+uint16_t AtomicExchange(uint16_t *a, uint16_t new_val);
 
 // Wrapper for TLS/TSD.
 void AsanTSDInit(void (*destructor)(void *tsd));