fix(spanner): avoid evaluation-order issue in function arguments (#9452)
fix(spanner): avoid evaluation-order issue in function arguments (#9452)

The behavior of
```
  extern void f(std::unique_ptr<T>, T);
  std::unique_ptr<T> p = ...;
  f(std::move(p), *p);
```
is undefined, so introduce a sequence point between dereferencing
the `unique_ptr` and moving it.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp