MSan: mark any memory allocated from the JS heap as uninitialized.

src/msan.h

Index: src/msan.h
diff --git a/src/msan.h b/src/msan.h
index 4130d22a65211166a5ffa3514733dac425f1155c..dfcbd239e14aa1dd7c0bd08f4fe04ed6450f2fd4 100644
--- a/src/msan.h
+++ b/src/msan.h
@@ -17,8 +17,17 @@
 # define MEMORY_SANITIZER
 #endif
 
-#if defined(MEMORY_SANITIZER) && !defined(USE_SIMULATOR)
+#if defined(MEMORY_SANITIZER)
 # include <sanitizer/msan_interface.h>  // NOLINT
+
+// Marks a memory range as uninitialized, as if it was allocated here.
+# define MSAN_ALLOCATED_UNINITIALIZED_MEMORY(p, s) \
+    __msan_allocated_memory((p), (s))
+#else
+# define MSAN_ALLOCATED_UNINITIALIZED_MEMORY(p, s)
+#endif
+
+#if defined(MEMORY_SANITIZER) && !defined(USE_SIMULATOR)
 // Marks a memory range as fully initialized.
 # define MSAN_MEMORY_IS_INITIALIZED_IN_JIT(p, s) __msan_unpoison((p), (s))
 #else