Fix asan/TestCases/Darwin/scrible.cpp to work on platforms where `l…

compiler-rt/test/asan/TestCases/Darwin/scribble.cpp

@@ -13,7 +13,14 @@ struct Isa {
 };
 
 struct MyClass {
-  long padding;
+  // User memory and `ChunkHeader` overlap. In particular the `free_context_id`
+  // is stored at the beginning of user memory when it is freed. That part of
+  // user memory is not scribbled and is changed when the memory is freed. This
+  // test relies on `isa` being scribbled or unmodified after memory is freed.
+  // In order for this to work the start of `isa` must come after whatever is in
+  // `ChunkHeader` (currently the 64-bit `free_context_id`). The padding here is
+  // to ensure this is the case.
+  uint64_t padding;
   Isa *isa;
   long data;