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GH-44491: [C++] StatusConstant- cheaply copied const Status (#44493)



### Rationale for this change

It'd be convenient to construct placeholder error Status cheaply.

### What changes are included in this PR?

A constant error Status can now be constructed wrapped in a function like
```c++
Status UninitializedResult() {
  static StatusConstant uninitialized_result{StatusCode::UnknownError,
                                             "Uninitialized Result<T>"};
  return uninitialized_result;
}
```

Copying the constant status is relatively cheap (no heap interaction), so it's suitable for use as a placeholder error status.

Added `bool Status::State::is_constant` which causes copies to be shallow and skips destruction. Also `Status::state_` is a const pointer now; this helps to ensure that it is obvious when we mutate state_ (as in AddContextLine).

### Are these changes tested?

Minimal unit test added. The main consideration is probably benchmarks to make sure hot paths don't get much slower.

### Are there any user-facing changes?

This API is not currently public; no user-facing changes.

* GitHub Issue: #44491

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>