fix: constexpr bit_cast with empty base classes (#82383)

Prior to this commit, clang would fail to produce a constant value for
`b` in:

```c++
struct base {
};

struct s : base {
    int z;
};

constexpr auto b = std::bit_cast<s>(0x12); 
```

e.g. https://godbolt.org/z/srrbTMPq4