[libc][Obvious] Add a file which was missed in 001a12e.

Author: Siva Chandra Reddy

libc/src/__support/sanitizer_annotations.h

@@ -0,0 +1,19 @@
+//===-- Convenient sanitizer annotations ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_SANITIZER_ANNOTATIONS_H
+#define LLVM_LIBC_SRC_SUPPORT_SANITIZER_ANNOTATIONS_H
+
+#if __has_feature(memory_sanitizer)
+#include <sanitizer/msan_interface.h>
+#define SANITIZER_MEMORY_INITIALIZED(addr, size) __msan_unpoison(addr, size)
+#else
+#define SANITIZER_MEMORY_INITIALIZED(ptr, size)
+#endif
+
+#endif // LLVM_LIBC_SRC_SUPPORT_SANITIZER_ANNOTATIONS_H