[llvm] [ADT] Refine MutableArrayRef forwarding constructor constraints (PR #183806)

[Jakub Kuderski via llvm-commits]

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kuhar wrote:

Your check is stricter than what the comment above says:

```suggestion
/// Construct a MutableArrayRef from a type that has a data() method that
/// returns a pointer to `T *`.
```

https://github.com/llvm/llvm-project/pull/183806