[libcxx-commits] [libcxx] [libc++] Create a `constexpr` hash to use during constant evaluation (PR #214946)

[A. Jiang via libcxx-commits]

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frederick-vs-ja wrote:

```suggestion
  [[nodiscard]] constexpr std::size_t operator()(std::nullptr_t) const noexcept {
```

Also we should include `<cstddef>` for `size_t`.

https://github.com/llvm/llvm-project/pull/214946