[sanitizer][Darwin] Suppress -Wno-non-virtual-dtor warning

Suppress the warning:
```
'fake_shared_weak_count' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
```

The warning has been recently enabled [1], but the associated cleanup
missed this instance in Darwin code [2].

[1] 9c31e126
[2] d48f2d7c

Differential Revision: https://reviews.llvm.org/D94139