Appease MSAN buildbots.

This is just a compile time test, but we have MSAN buildbots that will
fail since `exp` was uninitialized.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@225286 91177308-0d34-0410-b5e6-96231b3b80d8

test/atomics/atomics.general/replace_failure_order.pass.cpp

@@ -25,7 +25,7 @@
 int main() {
     std::atomic<int> i;
     volatile std::atomic<int> v;
-    int exp;
+    int exp = 0;
 
     i.compare_exchange_weak(exp, 0, std::memory_order_acq_rel);
     i.compare_exchange_weak(exp, 0, std::memory_order_release);