C++20 std::move(ss).str() first copies and then delete the original string instead of moving it

https://godbolt.org/z/44Ke34fGz
```cpp
std::stringstream ss("a very long string that exceeds the small string optimization buffer length");
const char *p = ss.view().data();
std::string s = std::move(ss).str();
assert(s.data() == p); // shouldn't be a second allocation
```
In C++20 and later, moving-out-of `ss` should move the string, not make a fresh copy.