LWG issue 2143: ios_base::xalloc should be thread-safe

llvm-svn: 192539
diff --git a/libcxx/src/ios.cpp b/libcxx/src/ios.cpp
index 754c2c9..e79b328 100644
--- a/libcxx/src/ios.cpp
+++ b/libcxx/src/ios.cpp
@@ -149,8 +149,11 @@
 }
 
 // xalloc
-
+#if __has_feature(cxx_atomic)
+atomic<int> ios_base::__xindex_ = ATOMIC_VAR_INIT(0);
+#else
 int ios_base::__xindex_ = 0;
+#endif
 
 int
 ios_base::xalloc()