[libc++] Introduce __debug_db_insert_c()

There are a lot of
```
#if _LIBCPP_DEBUG_LEVEL == 2
    __get_db()->__insert_c(this);
#endif
```

This patch introduces `__debug_db_insert_c()` to put the `#if` in one central place.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D116947