Clarifications for "sycl::exception_list"

This issue captures a number of problems with the definition of `sycl::exception_list`.

* We do not specify the kind of iterator for `exception_list::iterator`. Assuming we want to follow the exception list in the [C++ draft parallelism specification](https://isocpp.org/files/papers/N4352.html#parallel.exceptions.synopsis), we should define it to be `LegacyForwardIterator`. If we want `exception_list` to be a read-only container, then we should further clarify that the `iterator` type is a "LegacyForwardIterator to const value_type".

* It's unclear (to me) whether the spec requires an implementation to have a default constructor, a copy constructor / operator, or a move constructor / operator. Rather than relying on C++ rules for implicitly declared constructors / operators, I think we should explicitly list these in the synopsis and describe them.

Here is a proposed synopsis which addresses these issues:

```
class exception_list {
public:
using value_type = std::exception_ptr;
using reference = value_type&;
using const_reference = const value_type&;
using size_type = std::size_t;
using iterator = /* LegacyForwardIterator to const value_type */;
using const_iterator = /* LegacyForwardIterator to const value_type */;

exception_list();
exception_list(const exception_list &other);
exception_list(exception_list &&other);
~exception_list();

exception_list &operator=(const exception_list &other);
exception_list &operator=(exception_list &&other) noexcept;

size_type size() const;
iterator begin() const;
iterator end() const;
};
```

Note that I retain the class types `const_iterator`, `reference`, and `const_reference` mostly for compatibility with SYCL 1.2.1, which defined these types. Nothing in the API actually uses them.

Possibly, we do not want to support a default constructor since the SYCL runtime always creates `exception_list` objects. In that case, we should show the default constructor as deleted in the synopsis (or maybe just omit it).

Alternatively, we might decide that `exception_list` should meet all the requirements of `Container`. In order to do this, we would need to define additional types and member functions.

Start by reviewing the specification definition of sycl::exception_list and the linked C++ draft parallelism specification. Resolve the iterator, special member, and container-requirement alternatives, then update the synopsis and requirements so the supported interface is unambiguous.

Written by the indexing model from the issue text.