[TSan] Fix initialized and declared ‘extern’ warning

Avoid the following warning which fails a bot (via -Werror).

```
/tmp/gotsan.JfrpVPu7pG/gotsan.cc:10456:25: error: ‘_tsan_pointer_chk_guard’ initialized and declared ‘extern’ [-Werror]
extern "C" __tsan::uptr _tsan_pointer_chk_guard = 0;
                        ^~~~~~~~~~~~~~~~~~~~~~~
```

llvm-svn: 364823