This unbreaks C++20 buildbot that was broken since
402baea0a9.
With implicit conversion in C++20 compilation mode the spaceship
will unintentionally be based on `operator bool`:
```cpp
auto foo(Location L, Location R) {
return L <=> R;
// Equivalent to the following line due to implicit conversions.
// return L.operator bool() <=> R.operator bool();
}
```
The spaceship operator is rarely used explicitly, but its implicit uses
in the STL may cause surprising results, as exposed by the use of `std::tie`
in 402baea0a9, which ended up changing the
comparisons results unintentionally.
cf9374933d