[ADT] Conversion from ArrayRef<S:T> to ArrayRef<T> does not work

Conversion of an `ArrayRef` of some class to an `ArrayRef` of the base class of that class fails with the following error message:
```
ArrayRef.h(94, 28): Candidate template ignored:
requirement 'std::conjunction_v, std::is_integral>' was not satisfied [with C = mlir::ArrayRef]
```

### Example
```c++
ArrayRef x;
ArrayRef y(x);
```

Start by locating the ArrayRef conversion constructors and constraints in the LLVM ADT implementation, then reproduce the StringAttr-to-Attribute example from the issue. Done means the shown conversion is accepted and covered by a regression test for derived-to-base ArrayRef conversion.

Written by the indexing model from the issue text.