[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] 9c31e12609
[2] d48f2d7c02

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

compiler-rt/lib/tsan/rtl/tsan_interceptors_mac.cpp

@@ -438,6 +438,7 @@ struct fake_shared_weak_count {
   virtual void on_zero_shared() = 0;
   virtual void _unused_0x18() = 0;
   virtual void on_zero_shared_weak() = 0;
+  virtual ~fake_shared_weak_count() = 0;  // suppress -Wnon-virtual-dtor
 };
 }  // namespace